process_unicode: Add a way to enter unicode symbols by name

process_unicode: Add a way to enter unicode symbols by name

The purpose of this change is to allow keymaps to specify a dictionary
of unicode symbol name to code mappings, and let the person at the
keyboard enter unicode symbols by name.

This is done by having a way to trigger unicode symbol input mode, when
all keys are cached until Esc, Enter or Space are pressed. Once that
happens, we try to look up the symbol from our lookup table. If found,
we erase back, and type the unicode magic in to get that symbol. If not
found, we still erase back, start unicode input mode, and replay what
the user typed in.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>