month-view: add proper count of overflowed events

Keep a list of the hidden events per cell in order to add them later to
the popover
month-view: track events on overflow indicator area

We keep to state variables to handle hovering and clicking on the
indicator.
Note: This commit does not build because requires of upcoming work
month-view: initial impl of overflow popover
month-view: highlight overflow indicator on hover
month-view: fix popover placement on RTL locales
month-view: FIXME removed, feature added
month-view: update note about Gtk+ clipping bug
month-view: add title to overflow popover
month-view: preserve hovered cell on popover show

The hovered cell is the indicator to draw the background and the
coloring of the overflow indicator. Preserve its value as long as the
overflow is shown.
month-view: draw a background on the overflow indicator

This code suffer from a similar bug to ::queue_area on
::size_allocate. This need some revisiting to make it work properly.
month-view: read 12/24 time format setting

The setting will be used when showing the start time of events in the
overflow popover.
month-view: show start time header on overflow popover
event-widget: add ::compare_by_start_date

This function will eventually deprecate gcal_compare_event_widget_by_date()
month-view: sort single-day event widgets by start date

This guarantee that events widgets appear sorted by its start date on
the overflow popover