- Add DockManager::signal_dockable_unregistered

- Add MainWindow::on_dockable_unregistered(Dockable* dockable)
- Connect MainWindow::on_dockable_unregistered(Dockable* dockable)
to DockManager::signal_dockable_unregistered

- Add CanvasView get & set, popup and toolbar ui_merge_id
- Affect CanvasView popup & toolbar ui_merge_id on MainWindow::on_dockable_registered

- On MainWindow::on_dockable_unregistered, remove from App::ui_manager
CanvasView popup and toolbar ids.

Nota : If a canvas has an exported group, the closed exported group appear
in the window listing until all reffering canvas aren't closed ...
It's a kind of feature.