Add scan() to detect already open clients.

This is useful for restarting stwm.

However it crashes when run with a VTE terminal (even when the terminal
is closed before restarting).
Try to track down reason of VTE-related crash.

As it seems, VTE terminals (xfce4-terminal, roxterm, etc.) create 4
windows upon starting, but only map one of them to the root window.

This is not a problem per se, however upon closing such a terminal, only
3 of the 4 are destroyed (and one of the windows is destroyed twice).
This causes problems in scan(), since there is a window left that is
probably not supposed to be there.

=> need to find out why VTE does this, and what dwm does to solve this
   issue.
Distinguish between mapped and unmapped clients.

I think I've been doing this already before - but honestly I've changed
quite a lot, so I don't really know what's new here...
Fix error message upon success.

The error message was displayed at the end of a loop (when the loop does
not return upon finding the right window).
Problem is there was a 'break' instead of a 'return', so the error
message got displayed even upon successful deletion of the correct
client.

Move around config variables (might get moved to a separate config.h
file later), and arranged function declarations alphabetically.
Detect window state upon restart and map them correctly.

There is the XWindowAttributes that holds all important information
about a window; using this I can restore the pre-restart state.
Add config.h.

Update README.
Merge branch 'restart'.

Implements the restarting behaviour (more or less) correctly.