Remote machine screenshot [feature request] #644

This PR closes the issue #644 (closed) and #801 (closed)

We have implemented a simple gtk/cairo based screenshot and an RDP plugin based screenshot, it's possible to implement the same mechanism for each plugin.

In the case the plugin screenshot implementation does not exists, remmina will fall back to the gtk/cairo based.

Two contributors have also provided a patch that makes the profile editor notebook content scrollable.