Move the Image Selection Menu to GimpDockWindow
Move the Image Selection Menu from GimpMenuDock to GimpDockWindow. That is, if a dock window contains many docks then they will share the same Image Selection Menu. To do this we need to move around quite a bit of code. Move the "context", "dialog-factory" and "ui-manager" properties from GimpDock to GimpToolbox, GimpMenuDock doesn't need it any longer. Turn the GimpDock getters for these properties into wrappers that go to the GimpDockWindow properties. In some places, most notably GimpToolbox construction, we use the GimpToolbox values of these properties, but most of the time it works fine to just use the GimpDockWindow properties. GimpDock::setup() and set/get_aux_info() have also been moved to GimpDockWindow since the only aux info for docks was for the image selection menu. Also, we don't bother porting gimp_menu_dock_destroy() to GimpDockWindow, but we leave the code around. If this is a problem, it will show.
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