Dropdowntree v5

[PaskalevStoyan]

resolves: #2803 , #3059
discussion: N/A

Because the popup part of the DropDownTree component is a TreeView, some rendering changes and class renames have been made to the TreeView:

• k-group -> k-treeview-group
• k-top | bot | mid -> k-treeview-top | bot | mid
• k-item -> k-treeview-item
• k-in -> k-treeview-leaf
• the expand / collapse icons are now wrapped in a span with a k-treeview-toggle class:

<span class="k-treeview-toggle">
  <span class="k-icon k-i-expand"></span>
</span>
<span class="k-treeview-toggle">
  <span class="k-icon k-i-collapse"></span>
</span>

• the text of the item is now wrapped in a span with a k-treeview-leaf-text class :
• Treeview with expand / collapse icons:

<span class="k-treeview-leaf">
  <span class="k-treeview-leaf-text"> ... </span>
</span>  

• Treeview with custom icons:

 <span class="k-treeview-leaf">
  <span class="k-icon k-i-file"></span>
  <span class="k-treeview-leaf-text">...</span>
</span>

• Refactor treeview markup
• Introduce sizing options
• refactor present treeview visual tests
• Add kendo prefix to variables
• refactor states
• Add variable descriptions
• Add tests
• Add docs

[Juveniel]

@PaskalevStoyan we need a separate tracking issue for the TreeView, where the rendering changes are described.

[joneff]

There are a couple of issues, but we can merge and fix later.