Added selection toolbar item
Added basic implementation of drag-select area. The volume intersection test is not exact yet.
Revised the way converters work to make them more useful. I recommend deleting the FBXToglTF and ColladaToglTF scripts in Scripts/Start/Converters, if you have those files. Converters can now be enabled and disabled with settings. Everything is managed entirely in script, except for detecting files that need to be converted and emitting the convert file event.