keybindings: Only add multiple keycodes from the same level

The reason why multiple keycodes could be mapped to a single keysym was
to support having both KEY_FAVORITES and KEY_BOOKMARK map to
XF86Favorites. However, iterating through all layout levels adding all
key codes has severe consequences on layouts with levels that map
things like numbers and arrow. The result is that keybindings that
should only have been added for keycodes from the first level, are
replaced by some unexpected keycode where the same keysym was found on
another level.

An example of this is the up-arrow key and l symbol. Normally you'd find
both the up-arrow symbol and the l symbol on the first level and be done
with it. However, on the German Neo-2 layout, layout level 4 maps the
KEY_E to the l symbol, while layout level 4 maps KEY_E to up-arrow.
Which ever gets to take priority is arbitrary, but for this particular
case KEY_E incorrectly mapped to up-arrow instead of the l symbol,
causing the keyboard shortcut Super+l, which would normally lock the
screen, to trigger the workspace-up (Super+up-arrow) key binding.

https://bugzilla.gnome.org/show_bug.cgi?id=789300

src/core/keybindings.c

@@ -350,7 +350,9 @@ add_keysym_keycodes_from_layout (int                           keysym,
 {
   xkb_level_index_t layout_level;
 
-  for (layout_level = 0; layout_level < layout->n_levels; layout_level++)
+  for (layout_level = 0;
+       layout_level < layout->n_levels && keycodes->len == 0;
+       layout_level++)
     {
       FindKeysymData search_data = (FindKeysymData) {
         .keycodes = keycodes,