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.
Calculate event blocks upfront, in a way that is agnostic to
pixel sizes, and keep them alive as long the event is around.
Calculate which event blocks are visible at layout time, and
only set the child visibility of the event widgets, instead
of truly showing or hiding them. This makes size_allocate not
queue relayouts.
There are many opportunities for optimizations that we can
eventually land, but this seems to work decently so far.
RTL support is still missing.
