Better Collada Exporter fixes and Godot Export Manager Addition

Hi,
I made some fixes for the Better Collada Exporter Addon for Blender.
Read the commit logs for the list of fixes.

I also created a Godot Export Manager which uses the Better Collada Exporter to export user defined groups. The user can define customized export groups with objects assigned to it. All settings will be stored in the blendfile and the groups can be exported within seconds. On the one hand a export on blendfile save can be enabled. Or it can be exported on click. As the user prefers.