In month view and year view, when the mouse is hovering over
an event, nothing is shown. A tooltip with the event information
would be much more useful in this scenario.
Fix that by showing a tooltip displaying the event's title, time
and date, and, if present, the location and notes of the event. Long
notes are truncated and ellipsized.
https://bugzilla.gnome.org/show_bug.cgi?id=774952
