Draft: Refactor the drawer and the overview

This makes Phosh's design a bit closer to GNOME Shell's by:

• using a dash to store the favorites
• moving the overview above the home button
• using a dark gray color for the overview
• separating the overview and the apps list
• paginating the apps list horizontally

This will help implementing gestures later.

To do:

• limit the number of apps visible in the dash

CC @bertob @snwh