calendar: Grab key focus when user changes day

When the user changes the active day by mouse click or keyboard focus
plus key press on a day in the grid, always move the keyboard focus to
the newly activated day.

This basically restores functionality that was introduced in commit
31478e9fb441 but got lost again in the re-factoring in commit
cc4659f5c697.

https://bugzilla.gnome.org/show_bug.cgi?id=725606