Fix crash in Editor::drawOneSpriteUnclippedRect() if a temporary surface cannot be created (fix #413)
Add Alt+Shift+G shortcut to toogle the pixel grid visibility (fix #504)
Add "View > Show Pixel Grid" menu option to switch the pixel grid
Fix region_tests compilation (as now it depends on pixman library)
Add keyboard shortcuts customization (close #253)

Changes:
* Add KeyboardShortcutsCommand and window
* Add SelectAccelerator window
* Replace modules/gui.cpp functions with app::KeyboardShortcuts and
  app::Key with the logic to load/save/handle keyboard shortcuts
* Change ui::Accelerator concept: now it represent just one keyboard
  shortcut, not a set of shortcuts
* Remove ui::Accelerator from ui::MenuItem, now the key is associated
  in app::AppMenuItem and it's a app::Key
* Add Command::onGetFriendlyName() to get a user friendly name of the
  command depending on its parameters