made Layout a static struct in layout.c, added some convenience getters in layout.c, now lt->arrange accesses are not possible anymore, arrange() is the super-arrange function which sets up all layouts
/* See LICENSE file for copyright and license details. */
/* tile.c */
void addtomwfact(const char *arg); /* adds arg value [0.1 .. 0.9] to master width factor */
void tile(void); /* arranges all windows tiled */
void zoom(const char *arg); /* zooms the focused client to master area, arg is ignored */