Added separator comments between groups
Tidied up inconsistent indentation
Ensured that the application does not quit when timer is running

If the MainWindow is closed, a new hidden instance is created, so that
Gtk.Application does not automatically terminate itself due to a lack
of windows attachted to it.
TimerView can now be initialized with a running timer

The term "set_running (false)" assumed, that a new TimerView would
always correspond to a newly created and thus stopped TaskTimer. As the
timer now continues to run in the background, this is no longer true.
TimerView is displayed by default, if the timer is running

By default Go For It! presents its to-do list upon window instantiation.
If the user closes the window and the timer continues in the background
it is more likely, that she/he would expect the timer to be displayed
by default.
Resolved view dependency for automatically selecting next task

Until now automatically selecting new tasks worked "magically", because
the Gtk.TreeView widget has that behavior built in, as long as one
selection has been made before. Now, that MainWindow may start in
timer view, this cannot be guaranteed, and thus must be taken care for
else were.

The above changes also imply a behavioral difference: Not the next task,
but the first in the list gets selected. Assuming that users
arrange their tasks by priority, that makes more sense, in my opinion.