Fix T54094: Paint Palette saving presets path issues

Bumped version to 0.9.4
Some style tweaks
Rename some of the classes for consistency
Replace the import_module with register_class
Remove the Weights Property_Group since it is not used

Fix the issues with using related to relative paths for presets:
- use the os.normpath output of the absolute path
- add some checks for path existing
- add a warning about not saved blend file (bpy.data.filepath)