[dwm] Let's do some refactoring

Hi there,

Jukka proposed to introduce a mode container (I prefer calling
s/modes/layouts/ in the future) for easier integration of
additional arrangement algorithms in the future:

typedef struct Layout Layout;
struct Layout {
        void (*arrange)(void); /* arrange function, indicates layout */
        const char *symbol;
        Layout *next;
};

I like this idea and will adapt it with some slight changes
(beside putting all layout-related algorithms into separate C
files - like the master-related stuff of tiled layout, which
really only belongs to dotile()).

Regards,

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