Added formspec element 'proxy' for charbased input

This adds a new formspec-type that when focused, grabs keyboard input and immediatly
sends it as keycode. This will enable charbased input for mods. I want to use this
for ingame computers in dronetest-mod. Could also be used for lots of other stuff
like musical instruments, remote controls and everything that doesn't do with linebased
input.
it is used like this:
proxy[pos;size;name;texture;active-texture]

Also i changed the formspec-drawing to have a dirty-variable,
which gets set on change in focus. This enables the proxy element (and possibly others)
to have immediate visual reaction to focus change.