Set version number and release date in changelog.

Fix version number in developer's guide.

Add openTCS 5.2 (2021-03-29):

* New features:
  * For plant model elements' tooltip texts in the Operations Desk, sort
    properties lexicographically and colorize vehicles' states.

Add openTCS 5.1 (2021-03-24):

* Bugs fixed:
  * Made names generated for transport orders to be (really)
    lexicographically sortable.
* New features:
  * Add a `QueryService` to the kernel that can be used to execute
    generic/custom queries via registered `QueryResponder` instances.
  * Add support for creating plant models with multiple layers.
  * Add experimental support for peripheral devices, with device
    interactions triggered by vehicles travelling along paths.
    (Note that this is not really documented, yet, and that
    _experimental_ means that developers using any parts of it are on
    their own, for now.)
  * Add a new version of the XML Schema definition for the openTCS plant
    model.
  * Allow the scheduler to be triggered explicitly via
    `Scheduler.reschedule()`.
  * Show properties in model elements' tooltips.
* Other changes:
  * Split the Plant Overview application in two separate applications:
    The Model Editor provides model creation and manipulation
    functionality, while the Operations Desk is used for interacting
    with a plant while it is in operation.
  * Split the Operations Desk's pause button into a pause and a resume
    button.
  * Remove support for groups.
    (Layers can now be used to group plant model components.)
  * Allow project-specific edge evaluators and routing group mappings to
    be used.

Co-authored-by: Martin Grzenia <[email protected]>
Co-authored-by: Leonard Schüngel <[email protected]>

Add openTCS 5.0 (2020-06-05):

* Remove deprecated code.
  * Remove the TCP host interface kernel extension.
  * Remove the kernel application's GUI.
* `TCSObject` and its subclasses are now immutable and do no longer
  implement the `Cloneable` interface.
* Remove the JDOM dependency.
* In `BasicCommunicationAdapter`, use an injected `ExecutorService`
  (e.g. the kernel executor) instead of starting a separate thread for
  every vehicle driver instance.
* Add a new and cleaned up version of the XML Schema definition for the
  openTCS plant model and add new bindings.
* Update project to Java 13.
* Update Mockito to 2.28.2.

Co-authored-by: Martin Grzenia <[email protected]>
Co-authored-by: Mustafa Yalciner <[email protected]>
Co-authored-by: Leonard Schüngel <[email protected]>

Add openTCS 4.20 (2020-12-18):

* Fixes:
  * Default `Scheduler`: Properly handle requests for _same-direction_
    blocks for some edge cases.
  * Default `Scheduler`: Really free all resources when taking a vehicle
    out of the driving course.
* Other changes:
  * Plant Overview: Improve performance for vehicle state updates.

Co-authored-by: Martin Grzenia <[email protected]>

Add openTCS 4.19 (2020-07-02):

* New features:
  * As with paths, locations can now be locked via the Plant Overview
    application to prevent them from being used by vehicles.

Co-authored-by: Martin Grzenia <[email protected]>

Add openTCS 4.18 (2019-04-09):

* New features:
  * Provide the route to be travelled to vehicle drivers with every
    movement order, for cases in which vehicles require some information
    about it.
  * Allow supplementary configuration sources to be registered via
    service loader.
  * Allow a configuration reload interval to be set via a system
    property.
* Other changes:
  * Improve performance of loading a plant model file into the kernel.
  * Rename transport order category to transport order type.
  * Update Spark to 2.9.1.

Co-authored-by: Martin Grzenia <[email protected]>
Co-authored-by: Mustafa Yalciner <[email protected]>

Add openTCS 4.17 (2019-12-10):

* Bugs fixed:
  * In the Plant Overview application's "Continuous load" plugin panel,
    it is now possible to properly remove/delete entries in the drive
    order and property tables.
  * Changing the loopback driver's state through its panel in the Kernel
    Control Center application now works in all cases.
* Other changes:
  * When using the Kernel's RMI interface with SSL enabled, avoid side
    effects on other components using SSL.

Co-authored-by: Leonard Schüngel <[email protected]>
Co-authored-by: Martin Grzenia <[email protected]>

Add openTCS 4.16.1 (2019-10-30):

* Bugs fixed:
  * Fix creating links between points and locations in the Plant
    Overview application.

Co-authored-by: Martin Grzenia <[email protected]>