Custom actions are basically any action (currently GtkAction) which can
be assigned a shortcut. Now they can also be assigned to an input device
button (with modifier or not).
* The relative brush size change was not implemented anyway. Maybe later.
* Also changing the defaults to GIMP_MODIFIER_ACTION_BRUSH_PIXEL_SIZE
which I think might be the most understandable size variant.
* Finally re-label "Rotate" to "Rotate View" as per Aryeom's feedback
because just "Rotate" is indeed confusing as it could mean several
different actions in GIMP.
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.
Jacob Boerema