More export checks, bugfixes

Note:

This is still in beta and the behaviour may change between releases. Also bugs might still occur, which we'll try to fix as soon as possible. So feel free to report bugs and issues in the #w3d-blender-plugin channel on OpenSAGE Discord. Also see Troubleshoting for more information.

Changes:

• cancel export if a mesh and a bone share the same name and mesh is not configured properly
• inform user if both vertex bone weights do not add up to 100%
• Bugfix: handling of specular and emission color
• Bugfix: use proper file extension for loaded textures
• Bugfix: split vertices with n uv-coords into n vertices