Ability to expand dock into panel

### 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 https://github.com/elementary/dock/issues/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 https://github.com/elementary/dock/issues/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"

Start by reviewing the existing dock implementation and the related issues #337 and #511, then compare the Ubuntu Dock and Deepin examples linked in the proposal. Done means the dock can expand into a fixed-width panel with the application menu at one end and background-apps or workspace switching at the other, while launcher items remain distinct and predictable.

Written by the indexing model from the issue text.