Fix 517-After closing an exported Canvas it can't be reopened from Canvas Browser

- The canvas_view was hided, but never show again. Show it on Canvasview::present
+
- Some comments around
588-Window menu doesn't remove a closed document - progress

- Add DockManager::signal_dockable_unregistered
588-Window menu doesn't remove a closed document - progress2

- Add MainWindow::on_dockable_unregistered(Dockable* dockable)
- Connect MainWindow::on_dockable_unregistered(Dockable* dockable)
to DockManager::signal_dockable_unregistered
588-Window menu doesn't remove a closed document - progress3

- Add CanvasView get & set, popup and toolbar ui_merge_id
- Affect CanvasView popup & toolbar ui_merge_id on MainWindow::on_dockable_registered
Fix 588-Window menu doesn't remove a closed document

- 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.
Merge pull request #115 from d-j-a-y/djay_window_menu

Fix issue #588 - Window menu doesn't remove a closed document
Show render time in status bar (benchmark)
Display benchmark data for workarea