Add persistence and session restore for the RemoteDesktop portal.

[Adamskye]

Like with the ScreenCast portal, the RemoteDesktop portal allows for specifying persist mode and may return some restore data so that a program wouldn't need to prompt the user every time they want to use it. This should probably be implemented the same way that it has been for the ScreenCast portal.

[bilelmoussaoui]

A patch would be appreciated. Otherwise I will look into it the upcoming days

[Adamskye]

There is already a struct for PersistMode in screencast.rs, and I wasn't sure whether I should have reused this one or created a new one for remote desktop. I opted for the latter option, if that's okay.

[bilelmoussaoui]

This is now handled but let us keep the issue open till we add it to the demo as well