Add a screen sharing plugin

Add a screen sharing plugin

This adds a plugin called screen-share.so.  If the screen-share.so module
is imported, it will add the CTRL+ALT+s keybinding to start a screen
sharing session.  If you press CTRL+ALT+S, weston will spawn another copy
of weston, this time with the RDP backend, and mirrors the current screen
to it and adds any seats from RDP as aditional seats.  The current screen
is defined as the one with the mouse pointer.  Currently the CTRL+ALT+s
keybinding is hardcoded as the only way to activate screen sharing.  If, at
some point, shells want more control over the screen sharing process, the
API's should be easy to update and export to make this possible.

For security, the command and path to weston is currently hard-coded.  It
would not take much aditional code to make this configurable or to allow a
shell to launch other screen-sharing programs.  However, handling those
security issues is outside the scope of this patch so it is hard-coded for
now.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>