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 can be unfocused by mouse. It is based on a hidden editbox, so it should work 
with the software-keyboard.
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.
Please rename this.
Because C++ has the C standard as a normative reference the following applies:
> 7.1.3 Reserved identifiers
> All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.