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 often the window 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) until shortly after.