Fix for #4330

Fix for #4330

Since ec027bf dynamic resolution is broken when used with egfx. Before that commit
we were tracking a server sent resize by setting a DesktopResize callback. This callback
is called when the desktop is resized by the server. Anyway the problem was that when this
callback is called, the activation sequence is not always completed, which were leading to
some freeze with 2012r2 servers (sending packets before the sequence is finished).
So with the faulty commit, we are tracking server resizes by subscribing to the Actived
event, that is called at the end of a reactivation sequence, so we're sure to not send packets
when not fully activated.
Anyway the issue that shows on (#4330) is that when you use egfx, no reactivation sequence happens,
the server only sends a ResetGraphics message with the new size, and so we miss the resized event.
This fix introduces a new GraphicsReset event, makes the display channel subscribe to that event,
and react accordingly.