Add RDP support

The RDP support is powered by gtk-frdp which is powered by freerdp.

Gtk-Frdp (https://gitlab.gnome.org/felipeborges/gtk-frdp) is a small widget/library that I wrote to render RDP content with FreeRDP.

These patches make Boxes support RDP and also adds an option to disable the feature enterely.

meson _build -Drdp=false

A few notes about Gtk-frdp:

• It does scaling by default.
• It is a git submodule.
• Please file bugs on the library repository (unless they are obviously in Boxes).

To test it you can:

• Connect to an RDP server of your choice by going to [NEW] -> Enter URL -> rdp:// Or
• Use a Windows VM: make sure the network bridge is existent. Go to Windows' settings and enable remote connections. Or
• Use the https://www.secure-od.com/remote-desktop-access/ demo server (please be cautious)

Server address: 109.168.97.222
Username: demo2
Password: D3m02014Test*

Fixes #4 (closed)