Currently the menu has a hardcoded width which result into the hover
effect of the "Open Calendar" item being "cut off" in the middle rather
then reaching to the edge.
To be consistent with other menu items, make it expand to fill the available
space.
We try to position the boxpointer centered above the calendar,
which swaps position with the events list when using a RTL locale,
so make the menu alignment dependent on the text direction.
https://bugzilla.gnome.org/show_bug.cgi?id=642721
