Add OSC 52 sequence support to libvte

[fpqc ?? <harry gindi live com>]

Hi list,

The popular tmux software has implemented (in the master branch of its git 
repo) some new functionality making use of some older xterm functionality 
that hadn't been much used: 

The ability to use the OSC 52 escape sequence to make the tmux clipboard 
available to the outer terminal emulator.  This means that the tmux 
selection mode/clipboard can be shared back (even without any kind of 
forwarding) to the local terminal emulator, which can then use that to set 
the X or Wayland clipboard.

I have already implemented this in both the X and Wayland versions of 
suckless's st (simple terminal) terminal emulator.  I'd like to implement 
it in the GNOME libvte widget as well, but before I bother writing a patch, 
I'd like to know if this is a contribution you'd be willing to accept 
(perhaps allowing it to be enabled as a preference).  

Terminal Emulators supporting OSC 52 clipboard setting:
xterm
(u)rxvt
(patch pending approval on suckless-hackers mailing list) st (X version)
(available in the 'selection' branch) st (unofficial Wayland version) 
(https://github.com/michaelforney/st/tree/selection)
mintty (for windows)

Thanks for your help.

Cheers,

Harry