Implement screen cast portal

[leolost2605]

Supports everything (of version 3 i think at least) now and shows a dialog where you can select what you want to record.

The only thing I'm not really sure about is what I'm supposed to do with the VIRTUAL input source. IIUC this is just the whole virtual display across all monitors. Should this be shown as an extra entry under monitors or somewhere separate? I'm currently showing it separate as "Entire Display"

Todo:

• [x] set parent window
• [x] do something with virtual
• [x] Make the dialog look nice (Meh, help appreciated :))

Follow ups:

• [ ] Support session save and restore, i.e. upgrade the version to 4

[leolost2605]

I've tested this with Kooha and with mutter 46