Introduce framework for preferences
Change notifications behavior to boost speed; change notification-related preference names

In order to increase the apparent speed of the notifications (make them
look instantaneous), all previous notifications are removed before the
new notification is delivered. This way, only one notification (current
new song) is present at any time which makes the notifications appear
snappier. This also means that the notifications will be readable on
the lock screen (they won't group into one "Google Music - XX message"
notification), and therefore you won't need to unlock your computer to
control the music.

Also, preference names were updated to make them more readable and
sensible.
Add preference to enable/disable CSS stylesheet
Re-structure preferences window; small style change