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
Add a nix build expression, for development

This is a nix expression to build Synfig from source on any distro
where nix is installed. This will install all the dependencies from
the nixpkgs repo and build Synfig without interfering with the host
distro.

http://nixos.org/nix/

To quickly install nix, you can run the following command:

$ curl -L http://git.io/nix-install.sh | bash

To initialise it:

$ source ~/.nix-profile/etc/profile.d/nix.sh

To build synfig, from the current directory:

$ nix-build

To run the newly compiled synfigstudio:

$ ./result/bin/synfigstudio
Fix 608-Tool tooltip display "voidsymbol" if no accelerator is defined

- Add a test on accelerator presence before assign