Split header bar in a similar way to several other elementaryos apps

[jeremypw]

Problem

The design of Files differs from other elementaryos apps that have a primary sidebar (Code, Music, Calendar etc)

Proposal

Split the header and divide the header tools between them in a manner to be advised by the design team.

Prior Art (Optional)

Gnome Files, Elementary Code, Music, Calendar, SettingsPage

[danirabbit]

I had started playing around with mockups for redesigning the sidebar a while back:

Looks like GNOME Files includs some kind of menu and a search of some kind in the sidebar header area:

macOS Finder just includes window controls:

iPadOS Files has a hide sidebar button and some kind of menu: