Add update checks for managed platforms

[bmcclure]

Currently the version numbers detected for managed platforms may or may not be 100% accurate--they are what is reported in the platform's uninstaller entry in the registry.

There is a button which either says Install or Update when you select a platform, but currently in both cases it simply sends you to the download page for that platform.

There are a couple possible ways this could be used instead:

• Actually get the latest version from somewhere, and if the installed version is outdated, download a new release. This might not be ideal because most platforms offer self-update methods which are easier than re-installing.
• When Update is pressed, open the platform and automate the process of checking for updates however that is done