wibox.drawable: Redraw when moved

Let's consider the following notification:

  naughty.notify({ text = "foo", bg = "#88888888", timeout=1 })
  naughty.notify({ text = "foo", bg = "#88888888", timeout=2 })

When the first notification disappears, the second one is moved and takes its
place. However, its content was not redrawn. This makes the pseudo-transparency
break and the wallpaper that is shown in the back of the notification doesn't
match the real wallpaper.

Fix this by redrawing drawables when they get moved.

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