Multi monitor support
Add multi monitor support.
Flatpak is available for testing
Seems to work properly, the only issue I've is that with GTK (actually because of Xorg) is not possible to go full screen on some chosen monitor, either 1 or all, so if you are so "unfortunate" to have 3 or more displays and want to use only 2 for Remmina, you cannot.
Implements #376 (closed) to add multi monitor support
Replaces !2043 (closed)
CC: @hugbe8 @danielMd2805
Edited by Antenore Gatta (tmow)