Top Bar Organizer used to only interact with the panel, when the current
session mode is "user". However Ubuntu uses "ubuntu" as its session
mode, which made Top Bar Organizer not work.
Fix this by also interacting with the panel, if the parent session mode
is "user".
See here for some docs on session modes:
https://gjs.guide/extensions/topics/session-modes.html
The panel gets set to the state it should have for the unlock screen
before extension disable. Because of this the extension makes icons
show, which shouldn't show. So fix that by not ordering the panel, if
the current session mode isn't "user".
Also generally don't interact with the panel, if the current session
mode isn't "user".
Make sure unaccounted-for indicator containers (like the not ready app
indicator indicator containers) in the right panel box are on the left
of it, since the right panel box is logically right-to-left and to avoid
unnecessary shifting should they become accounted for.
Do this by not acting upon a new AppIndicator/KStatusNotifierItem item
immediately (as in trying to position it properly and maybe saving it to
settings), but rather act once the app indicators "ready" signal emits
by simply saving new items to settings and ordering the top bar boxes.
Let `BoxOrderManager` do the relevant work itself and get rid of
`AppIndicatorKStatusNotifierItemManager`.
This is also in preparation to make the addition of
AppIndicator/KStatusNotifierItem items work again.
Simplify `#overwritePanelAddToPanelBox` heavily by just calling the
original function in the overwrite and ordering the top bar and handling
new items afterwards.
Note that AppIndicator/KStatusNotifierItems are still not supported with
this refactor.
Introduce new `BoxOrderManager` to basically act as a heavy wrapper
around the box orders stored in settings, so that the other extension
code has easy methods to get, set and interact with box orders.
Refactor most of the code initially with it in mind.
Note that `#overwritePanelAddToPanelBox` needs more refactoring to
properly make use of that new idea (and it also needs refactoring in
general).
Also handle drag events in this function so connecting to "drag-end" in
widgets with "GtkDragSource" and a `globalThis` `ScrollManager` instance
aren't needed.