virtual-input-device/native: Try lower level keycodes first

Issue 405 relates to mismapped keys injected by gnome-remote-desktop.  It
seems to be because a keyval (keysym) may be in the keymap more than once.
We currently run through each keycode, trying from lowest level to highest
level and stop as soon as we find a hit, which means that if the key
symbol is in a lower level of a later keycode, we mis-pick the earlier one
with the higher level (hence the far less common greek_OMEGA rather than
common AT symbol).

This patch swaps the order of the loops (so we loop through the levels
from lowest to highest, and at each level run through the keycodes. We
make sure we only go as high as the highest level for all keys (by
figuring out the maximum on the first level's run through). This resolves
the issues being experienced and ensures it is possible to type characters
like @. It is slightly less efficient in the worst case, in that we keep
figuring out the maximum for each key multiplied by the maximum number of
levels (rather than 1), but in the common case is probably more efficient
(because there's a whole bunch of higher levels we never have to try if we
hit the right keycode first).

Fixes #405.

4 jobs for issue405 in 6 minutes and 10 seconds (queued for 1 second)