This PR adds an option to uninstall plugins in the Plugin Manager.
It also takes care of a few issues with the UI for plugin manager
itself. (Rapid multi-clicking of install button causing UI
discrepancies)
Plugins can now define variables PLUGIN_VERSION, which defines version of the plugin. It is only informative field for the user. PLUGIN_API_VERSIONS is a list of supported Picard versions. The versions doesn't have to match exactly, though. For example API version "0.9.0" will match Picard version "0.9.0alpha11". Use as specific version numbers as needed.