style: fixed some issues with the GNOME 3.10 theme
icons: Added a 256x256 icon (png and svg) which looks better in gnome-shell than 48x48
accessibility: Improve the accessibility around the application.

Now the AlmanahTag accept the focus and export the action "remove" with the
implementation of the AtkAction interface.

The main window now have access to the UI manager accellerators.