[dwm] Tabs in dwm

I have modified the restack() function to raise the sel client (it
won't happen when the focus changes moving the mouse, it would be
annoying). This adds some advantages and removes 3 or 4 loc.
This could be useful for people who wants a tabs functionality in dwm.
I will explain it with an example:
1. you have three terms open.
2. make the zoomed term float.
3. open a new term.
Now you can change focus between the two apps sharing the zoomed
space. I did this change to main dwm because it was annoying for me
not being able to use my tiled windows when I had a maximized float
one in front of them, but I'm finding this new "feature" really
useful. I thought somebody could find it interesting, and it could
even go in main dwm if arg agrees (though I don't think it so, and I
will respect his opinion).
I also implemented a new function to know if a client is hiding
another one, though it adds complexity, it is not really very useful
after using it for some time, and it was using the square indicators
in the title which now indicate the untiled/maximized state. So I will
get rid of it.

BTW: I like the changes introduced in the last dwm releases, good work arg!

See you,

-- 
- yiyus || JGL .