ScreenCast: Inhibit notifications when screen casting

Part of https://phabricator.kde.org/T13570

This prevents potentially unwanted messages to pop up while screen casting.

A user can overrule this and re-enable notifications any time by unchecking the do not disturb checkbox in notification applet.

This is Wayland-only. On X I think this is a lost cause since any app can just grab the screen. Most apps have some sort of control panel that is visible while recording, but they either have poor metadata or are override redirect and not managed by KWin. While I did a PoC patch for a "do not disturb" window rule and KWin scripting interface, I don't think this is a reliable enough solution... especially since Wayland gives us a proper one.

TODO:

• The portal seems to leak the Session when the client doesn't properly close() it. Am I not understanding correctly how this is supposed to work, or is this a bug? Can be easily reproduced with https://invent.kde.org/libraries/xdg-portal-test-kde by starting a screen share and then just closing the window.

@grulich @vkrause @davidedmundson