Use C-c as prefix for mode specific keybindings.

C-c should be used for mode specific keys and not C-x.
See "(elisp) Major Mode Conventions" for more info.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Add menu to WeeChat mode.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Fix `weechat-replace-input' called with nil.

If input ring is empty it gets called with nil which causes an error
in `insert'.  So replace it with "".

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>