Convert context menu to use Gio::Actions.
Reduce number of options depending on which item is right-clicked. Ensure that right-clicked item is in the selection.
- Change hide/lock attributes from Window level to App level and move code inside actions-hide-lock.cpp.
- Add new hide/lock Gio::Actions.
- Add new Layer Gio::Actions.
- Add new "object-set-clip-group" Gio::Action.
- Add action to wrap a selection by an anchor element (uses same code as for grouping a selection).
- Add action to open externally linked image for editing.
Note that "Layer" context menu is only used by the "Layer and Objects" dialog.