Per discussions, this PR restructures the library window navigation to take on a more conventional and macOS-like sidebar layout. The new sidebr conforms to Apple's HIG.
The sidebar is a much better navigational element than the segmented control in the titlebar for a few reasons:
A left-to-right hierarchy of master->detail is not only more obvious but is also ingrained in the layout of most other applications. So it will be more familiar to users and is more in keeping with how 90% of existing applications work on macOS (and on other OSes too, I believe).
Leaving the title bar to perform view-specific actions (e.g. grid vs list view, search) creates a better distinction between what elements in each application area relate to and how they act (navigation in sidebar, view-specific actions in the titlebar).
We have (had) a lot of controls in the top bar and yet have masses of horizontal space available in the library window. Making more use of horizontal space for navigation and cleaning up the top bar makes the top bar feel less cluttered and makes the main library view feel less over-sized
This is an initial iteration meant to translate existing functionality into a new sidebar and ensure that all basic new additions work. This will change, and several more entries will be added to the sidebar as we restructure parts of the library to make use of the additional space provided by the sidebar.
Signed-off-by: Claudio Cambra firstname.lastname@example.org