Add a click gesture to detect when we have to start monitoring
cursor motion. Also add a motion event controller to react to
cursor motion, and enable and disable the this motion controller
on button press and release, respectively.
Selection is implemented as a pair of GDateTimes stored in the
month view struct. Whenever a selection begins, the start date
is set; the end date is only set when moving the cursor. It is
valid to have a start of the selection, and no end (meaning, a
1 day selection); it is, however, forbidden to have an end but
not a start.
Future improvements can be added later, such as scrolling when
moving the cursor close to the edges of the month view. We're
gonna have to live without that for now though.
Implement the GcalView.clear_marks vfunc now that we have a
selection to operate on.
Philipp S