Replace RDP plugin with a new plugin that uses FreeRDP as a library

Currently the RDP plugin uses xfreerdp as separate executable and embeds it using some X trickery. This does not work on wayland and has no chance to work on wayland. Instead, we can use FreeRDP directly as a library and use it to render to a widget directly. In addition, while we're at it, we can modernise some parts of the RDP support like adding support for the graphics pipeline extension.

Things still to do:

• Wallet integration
• Implement all the configuration options
• More extensive testing of different RDP servers