Support for dead keys and compose sequences on Unix, by instantiating

a GtkIMMulticontext and having that filter most keypresses. I think
I've got this right so that it doesn't break any previous deliberate
keyboard-handling behaviour that's now _after_ the 'if (filter
keypress) return' statement.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9567 cda61777-01e9-0310-a592-d414129be87e