[dwm] persistent tags and static Layout definition

Hi there,

yesterday I pushed certain changes. The Layout struct is now
defined statically in layout.c and only used there. I added some
convenience methods to get access to specific things in layouts,
they are isfloating(), isarrange(func) and getsymbol().

I also added a new window property called __DWM_TAGS, which
remembers all tags of a window during restarts of dwm - but I
haven't tested this yet. Gottox told me in the IRC that clients
disappear with hg tip during restarts, I will check that later
today.

Have fun,

-- 
 Anselm R. Garbe >< http://www.suckless.org/ >< GPG key: 0D73F361