[dwm] focus issues

Hi,

There are a couple of focus issues in dwm-0.5:

- when selecting an empty view, focus stays on the last focussed
client. This means that keyboard input goes to a client that is not on
screen (and not even tagged with the selected tag).
- when lowering a client, it keeps focus, even if the pointer is in
another window after the lowering; the new top window the pointer is
in should get focus here.
- when you MODKEY-Button[13] click a window really fast, you can get
stuck in move/resize mode; wmii had this for a while too.

The attached patch fixes these issues; this will probably end up in
mainstream too when Anselm returns, but for now I'm sending it here,
in case other people find these issues just as annoying as me and
don't want to wait for a week.

Greetings, Sander.