keygrabbing: only look in the appropriate columns when resolving keysyms

keygrabbing: only look in the appropriate columns when resolving keysyms

Fixes: #585
This commit fixes the problem of i3 wrongly grabbing/interpreting (!) some key
bindings. Basically, when you have, say, "bindsym Mod1+4 workspace 4", but you
also have "bindsym Mod1+semicolon focus right" (both are default), and your
keyboard layout has semicolon on Mode_switch + 4, the "workspace 4" keybinding
was shadowed by the "focus right" keybinding, because that also resolves to
semicolon.

So, from now on, i3 will only consider column 0 and 1 for normal bindings and
column 2 and 3 for bindings using Mode_switch (columns as seen in xmodmap
-pke).