[43] window-tracker: Emit 'tracked-windows-changed' on title changes

This means the screen share window view gets updated also when the title of a window changes. This is important since it often changes shortly after mapping, which would otherwise go unnoticed by xdg-desktop-portal-gnome.

An example is launching Files and it showing up as 'Loading..', or launching a terminal, and it not showing the proper title (current directory), but some place holder that is never visible on the application window.

Adding it to the window tracker instead of in introspect.js itself is for convenience - there is no per window signal tracking there, and it already listens to the signal emissions about changed windows.

Part-of: !2578 (merged)

(cherry picked from commit a1d650ce)