Added close_on_mouseleave option

[proxict]

With this option, the window will close only when the mouse leaves the calendar window. There is a similar option close_on_unfocus which closes the window when it loses focus. The problem is that the window gains focus when it gets created and when the user first drags the mouse pointer over other windows before reaching the calendar window, the calendar closes. This happens when the window manager is set to automatically focus windows when the mouse pointer hovers over them.

[proxict]

Darn it, I was too quick with the PR as when testing it further, the window receives the leave-notify-event also when moving the mouse over the widgets inside of the calendar window.

EDIT: Hopefully fixed.

[dmedvinsky]

Looks good (for someone who doesn't know much about GTK 😬). Thanks!