Add in GtkApplication and GimpConsoleApp and GimpApp
This adds in the GimpConsoleApp and GimpApp and the GimpCoreApp interface to clear the path for the transition away from GtkUIManager and GtkAction as proposed in #7649 (closed)
This code fully works, however, however, it does replace any deprecated classes.
Note Just to be clear, fully based off work @nielsdg started!
Edited by Lukas Oberhuber