Fixed multiple shortcuts bugs

1) Standard actions, like Copy/Paste are now propertized as well.
   Otherwise, their custom shortcuts were never used.

2) ActionInfoItem now has a special field showing that the action
   is explicitly reset to null. This way we will not break it
   accidentally, when refactoring empty strings in the future
   (like it happened this time).

3) Split up KisActionRegistry and KisActionsSnapshot. The former
   is a singleton and used for propertizing the actions, but the
   latter one is just a snapshot used for filling up the settings
   dialog.

BUG:373184,372198