Support window capturing on Linux+Wayland

This issue is specific to Linux when running Wayland.

Currently we only support capturing screens on Wayland, with no option to support window-capturing. The implementation relies on a content-picker, and currently this only allows you choose a screen. The backend implementation we use has a very simple API to enable window capturing: https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.ScreenCast.html.

While enabling this should be straight forward, I attempted to make this patch. However, there was some issues related to the resolution of the captured video, making the resulting output partially corrupted. This should be possible to solve with some minor fixes.

If we allow this, we make it possible to capture windows on Linux+Wayland. Ideally, this would happen through QWindowCapture instead, but our support for content-pickers is generally slim at the moment. It should be possible to enable this through QScreenCapture.