keybindings: Do a breadth first search in our keysym to keycode code

Commit 1af00333 made a subtle change
regarding how XKeysymToKeycode behaves. It does a depth first search
while XKeysymToKeycode is documented to do a breadth first search:

"this function looks in each column of the core keyboard mapping in
turn and returns the lowest numbered key that matches in the lowest
numbered group" - from the XKB library documentation

Looping over all keycodes for each layout and level index makes us go
back to the previous behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=737134