Port to FreeRDP3

With the release of FreeRDP 3.0.0-rc0, the API in FreeRDP3 is now considered to be stable. The final 3.0 tag is expected to happen next week or the one after that after some further testing. FreeRDP2 will then (slowly) become unsupported too (maybe a final release, in case there are some commits without a tag). So, projects using FreeRDP2 should be ported to FreeRDP3.

Some noteworthy changes for the client side with FreeRDP3:

• Reworked internal state machine of the connection sequence (to be more correct with respect to [MS-RDPBCGR])
• Support for AAD authentication (for MS Azure)
• Support for RDSTLS (happens behind the scenes)
• Support for Kerberos
• Opus decoding support (helpful for distros, where FreeRDP is not compiled with FFMPEG support (like Fedora and Ubuntu)). It should be noted though, that Opus is a custom addition (MS Windows RDS does not support it, but g-r-d will do it with gnome-remote-desktop!216 (merged)). So, for Linux <-> Linux connections, where the client side uses Fedora or Ubuntu, this allows here a similar audio experience as with AAC.
• The settings structure is now opaque. It is not accessed directly anymore, but rather via getters and setters.