[Azoth MuC] Moved /kick and /ban here from Azoth core.
[Azoth] Added CoreCommandsManager w/ /clear command for #1668.
[Azoth] Instantiate CoreCommandsManager for #1668.
[Azoth] ChatTab::clearChat() renamed and made public for #1668.
[Azoth] Fixed includes in iprovidecommands.h.
[Azoth] Consider also core commands manager on message processing.

This also finally closes #1668.
[Azoth] Added basic /help command.