clock: Fix calendar popup positioning

Balló Györgyrequested to merge
City-busz/gnome-panel:calendar-popup-position into master

This fixes the problem that the calendar popup jumps into wrong position in some specific cases on update:

  • When the position is right or bottom, use the window size rather than the preferred size to calculate the distance from the panel.

  • When the position is right or left, always use north gravity.

Merge request reports