xmodmap: use Caps Lock as Mode_switch
xmodmap: swap Alt_L and Super_L

The modifier key for i3 is Mod1 (Alt) because the physical key(s)
Alt_{L,R} are easier to reach than Super_{L,R}. Some  applications
however use mappings with the Alt keys, but since i3 takes precedence
some of these mappings are not effective.

Swapping Alt_L and Super_L solves both problems.