Correctly handle Mode_switch/ISO_Level3_Shift (Thanks bacardi55)

Correctly handle Mode_switch/ISO_Level3_Shift (Thanks bacardi55)

I tested this with the following experiment:
$ setxkbmap 'us(intl)'
$ xmodmap ~/configfiles/midna/Xmodmap
$ xmodmap -e 'keycode 38 = a A adiaeresis Adiaeresis o O'
$ xmodmap -e 'keycode 49 = ISO_Level3_Shift ISO_Level3_Shift ISO_Level3_Shift ISO_Level3_Shift'
Then, Mode_switch + a yields ä, but ` + a yields o.

In i3lock, these were swapped (Mode_switch + a yielded o, while ä was not
reachable at all). The comment in the code explains it

(See http://code.stapelberg.de/git/configfiles for the Xmodmap)