The goal of this function is to give the focus to the active image
display. This is implemented as a core GimpDisplay virtual function
(with the actual implementation in GimpDisplayImpl), allowing to be used
even in core code, without actual GUI code (this was not necessary right
now, but I think it will be useful in future use).
This function is now called from the toolbox code (cf. 2 commits
earlier), avoiding code duplication. I also added a usage at the end of
toolbox_paste_received() so that a newly opened image by middle-click
paste in the toolbox directly gains focus.
GimpDisplay contains only the ID logic and the "gimp" and "config"
pointers, and lives in the core.
GimpDisplayImpl is a subclass and contains all the actual display
stuff. The subclass is only an implementation detail and doesn't
appear in any API.
Remove all hacks which pass displays as gpointer, GObject or
GimpObject through the core, or even lookup its type by name,
just use GimpDisplay.
Jehan