On OSX, call [NSApp activateIgnoringOtherApps] when a plug-in dialog
is shown, so the plug-in process becomes the active app, and the
dialog gets focussed.

In order to avoid doing this in GimpDialog (which is also used in
the core), do it in gimp_ui_init() which all interactive plug-ins
call, and when gimp_temp_proc_run() is called interactively, to
catch repeated activation of an already running plug-in.

Also, set GimpDialog's initial position to GTK_WIN_POS_CENTER,
or they will pop up in the top left corner.

Inspired by patches from Simone Karin Lehmann and Daniel Sabo.