This object's goal will be to manage customized modifiers per input
device button, which is why I add it to GimpDisplayConfig. It is in its
own new config file (`modifiersrc` in config dir) because it requires
GDK types access (well I could have done without, but it would have been
less semantic, hence not as good of an API). Anyway it is only useful
when running GIMP as GUI.
The GUI widget and the usage code to make this actually useful will come
in upcoming commits.
