Commit 086ae77929 had broken dnd of colors from toolbox's color area on
Wayland. Clearly Wayland did not like we changed focus on a click,
breaking the drag.
To fix this, do not propagate button press and release events from the
GimpFgBgEditor editor anymore. Yet, since changing colors is usually to
be used (often immediately) on the canvas, giving back the focus to
canvas still makes sense. Therefore, instead of using press/release
events, add semantic signals to GimpFgBgEditor: color-dropped,
colors-swapped and colors-default (additionally to already existing
color-clicked). Then connect to these new signals to grab focus for
canvas when relevant.
Thanks to Massimo for raising the broken color dnd feature.
d4733e5b21