[dwm] 3 patches: very small cleanup; merge focus+focustopvisible; layout.c cleanup

Hi,

attached are 3 patches:
-clean:
        small cleanup of layout.c for loop in floating() - to have it same
        styled as loop in tile()
-focusmerge:
        merges function of focus() and focustopvisible() into one function
        focus(). It has parameter Client *c and this behaviour:
                - c == NULL: if selscreen then select topvisible window else
                  deselect sel (used in leavenotify, while pointer goes to
                  another screen in multihead setup).
                - c && isvisible(c): focus that client

                - else: select topvisible client
        this makes behaviour of focus(NULL) more intuitive, the function do
        the best to select the client AND everytime has some client selected
        - if there is someone visible (and if selscreen, of course :). Also
        allow some cleanup in layout functions, too;
-un_ban:
        refactor (un)banning of (in)visible clients from layouts functions.
        Their code is then more readable and it ease creation of another
        layouts, too

--anydot