For users with git and GitPython installed, a new button is added to the
details view allowing them to change the branch they are on. Some other
minor UI changes are made to accomodate this new behavior.
Migrate to a UI that looks more like other software package managers, giving
more space to each addon by hiding the list when an addon is selected, and
providing a "back" button to get back.
Implements a two-style view option for the list of addons: expanded and
condensed, via a delegate that provides the drawing function for each row in
the table based on two different widget designs.
