main: Initialize brightnessManager before panel

main: Initialize brightnessManager before panel

The brightness indicator initialized via
`QuckSettings::_setupIndicators()` as part of the panel initialization
depends on `Main.brightnessManager`, which was only getting initialized
after the panel in `_initializeUI()`.

In practice this usually was not a problem, because the wait for
NetworkManager and Bluetooth in `_setupIndicators()` was enough of a
delay for `Main.brightnessManager` to have been initialized by the time
the brightness indicator is initialized. But on systems where
gnome-shell was built without these, this could have resulted in quick
settings not initializing properly.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/work_items/9378
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4401>
(cherry picked from commit 1e6e40e2a7e48ff1d319ba110de756a14c1dd550)