Initial support for Wayland and Mir

Allow remmina to be run under native wayland. Tested under Arch Gnome on Wayland, with

GDK_BACKEND=wayland remmina

• Plugins which use GtkSocket will report a correct error and will never be able to run
• Remmina still need to be linked to Xorg libraries for some extra functions (ie: keyboard mapping in RDP)