app: Make positioning of new dockables depend on window mode
Add GimpWindowingStrategy with create_dockable_dialog() and use it in dialogs_create_dockable_cmd_callback(). There are two implementations: GimpSingleWindowStrategy and GimpMultiWindowStrategy. Depending on the window mode, we want new dockables to appear in different places when created. In single-window mode, they should appear inside the single image window. In multi-window mode, a new dock window is created.
Showing with 605 additions and 49 deletions