Implement standards-compliant chat input editing functionality

Implement standards-compliant chat input editing functionality

Contributed by terencehill.

This, among other things, implements Key_Parse_CommonKeys, which
replaces Key_Console and unifies key handling code.

It's now possible to:
* move cursor to an arbitrary position with rightarrow/leftarrow and insert text
* paste text with ctrl-v
* enable insert mode with the Insert key
* clear the whole message with ctrl-u
* move to the beginning/end of the message with home/end
* move to the previous/next word with ctrl-rightarrow/leftarrow
* move to the previous/next character ignoring colors with shift-rightarrow/leftarrow

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12754 d7cf8633-e32d-0410-b094-e92efae38249