compositor-rdp: Add an option to disable client initiated resizes

This patch adds an option to the RDP compositor to disable
desktop resizes initiated by RDP peer. The current behaviour
is that if an incoming RDP peer suggests a resolution that is
not the current one, a mode_switch() is done and the desktop is
resized to that new resolution. This new flag allows to disable
that behaviour. If the flag is set, the new behaviour is that the
RDP peer will be instructed to resize to the size of the desktop.
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.