Simplify debugging custom style files

[MisterAI]

In order to support the creation of user defined type it would be helpful to get some debug output whenever loading or applying a custom style file was loaded. Otherwise it is hard to know why the extension doesn't behave as expected. Maybe there is already such a debugging possibility but I couldn't find any so far.

Possible implementation:

1. User sets some CustomFileTypes.xml in the settings.
2. User starts debugging session
3. In the output window at "Debug" an error message appears, if applicable e.g.:
   • Graphical Debugging: Error loading type "NameSpace::Of::SomeC$lass"; invalid characters
   • Graphical Debugging: Didn't load type "NameSpace::Of::AnotherClass"; already defined

[awulkiew]

Yes, optional logging should be there.