[dwm] Combining click targets

The attached patch lets you specify multiple click areas in a single
element of config.h's buttons[] array. For example,
ClkWinTitle|ClkStatusText, or perhaps ~0.

I find it useful. The downside is that it makes the enum harder to
read. Good enough for inclusion in the mainline?

Unrelated: I also removed the LeaveWindowMask, 'cause it doesn't seem
to be used.