* glom/mode_design/print_layouts/window_print_layout_edit.[h|cc]:
Implement the edit menu, enabling/disabling the menu items based on
whether something has been selected or previously copied.
* glom/utility_widgets/canvas/canvas_editable.[h|cc]: Added
get_selected_items() here, making it virtual. The override in
Canvas_PrintLayout is the only real implementation so far.
Added remove_item().
remove_all_items(): Emit signal_selection_changed if something
was selected.
* glom/print_layout/canvas_print_layout.[h|cc]:
Added remove_canvas_layout_item().
Removed update_layout_position_from_canvas(), putting it in
Canvas_LayoutItem instead.
* glom/print_layout/canvas_layout_item.[h|cc]:
Added update_layout_position_from_canvas().