Fix crash when selecting text brush

The patch does several fixes:

1) Removes KoEphemeralResource CRTP class and replaces it with two virtual
   functions in KoResource: isEphemeral() and isSerializable() to distincs
   between the two mixed uses of KoEphemeralResource. Non-serializable
   resource is the one that has no load/save() methods. Ephemeral resource
   is non-serializable, but also has no md5sum, therefore cannot be stored
   in the resources database.

   The main disadvantage of CRTP pattern is that IDE cannot parse such
   hierarchies, that is, such classes are not included into Ctrl+Shift+T
   report.

2) Removes `requiredBrushFilesList` .kpp tag that was used for the masking
   brushes in Krita 4.x. Modern Krita can fetch such list itself using
   KoResource::requiredResources().

BUG:443308