Add common Linux key combination methods to rfb

Add methods to send common Linux key combinations to VNC like ctrl+alt+Fx, alt+Fx and ctrl+alt+backspace.

I am not sure if this pr is worth merging into master but I had to create the methods for a project of mine and maybe someone else is interested in it.
It makes adding Linux key combination buttons to a noVNC page very easy.

For my project I have added them as overlay to a separate file so it is no problem to update noVNC automatically.