Ability to expand dock into panel

[teamcons]

Problem

Any non-launcher item is at an unpredictable place. Using non-pinned launchers, changing pinned launchers, causes everything to be displaced and shifted, and activating workspace switcher, background apps item, and possibly application menu, requires a conscious effort to look and point on to the correct location.

Thus these functions cannot rely on muscle memory and are potentially flow breaking. In the case of the application menu you cannot just throw the mouse in the corner, which is my concern for the latest design for #337

Proposal

Add the ability to expand the dock into a fixed-size full-width panel. Application menu at the start and background-apps / workspace switcher at the end

This way non-launcher items stays at a predictable fixed position and in a clear distinct place away from launcher items, making them also visually distinct and giving them also simpler access

this would also fix #511

I am open to doing a PR with some draft for this

Prior Art (Optional)

Ubuntu Dock expands the gnome dash

Deepin has the "Efficient Mode"

[kurisubrooks]

I created some art for a "Taskbar" version a while back, which merged the Dock and Wingpanel, but that might be too far outside of elementary's design scope/direction.

[danirabbit]

I was thinking about this the other day. Maybe we can expand the dock when hide mode is set to never? That would also solve an issue of weird space on the sides of the dock when a maximized window is there