desktop-shell: Properly handle seat hotplugging

Previously, desktop-shell would only create its internal shell_seat object
for each seat available when the desktop-shell module is loaded.  This is a
problem any time seats are created dynamically.  In particular, the Wayland
and RDP backends create seats on an as-needed basis and they weren't
getting picked up proprely by desktop-shell.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=77649
2 files changed