Redesign the sidebar/navigation to simplify the UI

EDIT: original issue was edited in light of the discussion (esp. discussion at https://governance.funkwhale.audio/d/m9LlGf4T/redesign-of-the-sidebar-navigation).

What is the problem you are facing?

I don't think the sidebar is really needed. All the items in it could be placed somwhere else, and I think that would make the UI clearer. Here's a mockup I made:

Here's where all the items in the sidebar would go:

1. "Logged in as", "Add content", "Settings" and "Log out" would be moved to a small user panel at the right end of the top bar. That pane would also include the source selector (my libraries / local libraries / etc) discussed in #591 (closed).
2. "Notifications" would be moved to an indicator right next to the user icon.
3. "Browse library" would disappear, since it already appears twice on the top bar.
4. "Favorites could disappear if we make favorite items appear at the top of the screen, as shown in #591 (closed) and here:

5. "Playlists" would be moved to the top bar, possibly next to "Radios".
6. "Activity" could also disappear, since the main screen would already be showing the recent activity (see #593 (closed)).

What are the possible drawbacks or issues with the requested changes?

I really think this layout would be simpler and easier to figure out, but some users could be confused by the change. Also, we would have to think of a new placement for the player and the queue.