When clicking the window icon, we now pop up the window menu at the
bottom left of the icon, no matter where you clicked it.
Right-clicking the title bar still pops up at the event position.
This patch adds a specialized window type for notifications. They now
have a title bar on the right-hand side, with a close button.
This removes the need for the "Done" button in notifications, giving us
a bit more horizontal space overall.
Design based on a mock-up from @xTibor :^)
