gui/window: Set parent widget as last focus when event widget is unfocusable

When selecting an event with a pointer, the keyboard-focused widget was being set instead of the last focus widget, which was then refocused after the event popover closed.

This patch fixes that bug by only setting the parent widget as the last focus widget if the event widget itself is unfocusable, like in the agenda view. The GtkWidget:focus-on-click property is already enabled, so it takes care of refocusing the event widget if it's focusable.

Commit a3fb5793 technically fixed #915 (closed), but this patch solidifies the commit.

Closes #915 (closed)