wibox.widget.systray: Properly work when rotated (FS#1115)

The systray widget's fit() function worked in the (rotated) user coordinate
space while the draw() function used device coordinates (unrotated). This meant
that width and height were swapped up in the calculations and the systray ended
up being way too small.

Fix this by making the draw() function use user coordinates, too. This means
that it needs some new magic to detect a rotated coordinate space. This, in
turn, means that the systray is now automatically rotated when you put it into a
rotate layout.

This might cause some minor breakage because people no longer need to call
:set_horizontal() on the widgets.

Thanks a lot to Alexander Gehrke for his help!

Signed-off-by: Uli Schlachter <psychon@znc.in>