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.