Add application window as video input

Broadcasters are currently given the option to use their entire screen as a video input. This is done with Electron's `desktopCapturer` API.

This same API offers the ability to capture specific application windows as video inputs too.

Users should be given the ability to toggle between "screen sharing" and "application window sharing".

Start by locating the existing Electron desktopCapturer flow that supplies full-screen video input and the UI that selects it. Trace how capture sources are presented and verify that users can toggle between screen sharing and application window sharing, with a selected application window working as a video input.

Written by the indexing model from the issue text.