This commit makes it possible to add titlebars to a client. These titlebars are
drawables.
The drawin's input handling is moved to the drawable. This allows it to use the
same code for drawin and titlebar input handling, although there are lots of
differences between the two on the C side.
On the lua side, a new wibox.drawable module is created which handles all the
drawable-specific magic and which can also be used for titlebars.
Signed-off-by: Uli Schlachter <psychon@znc.in>