Added support for writing MP3 files
preferences: use a single page to show all the preferences
show a message bar while fetching the disc information
properties dialog: use a GtkInfoBar to show messages
added the code to cancel the cover image fetching
download only 5 albums when searching for album title

instead of 10, to speed up the searching action.
fixed merge errors
gears menu: added keyboard shortcuts
application: reuse the current window if available
properties: use symbolic icons for the navigation buttons