[UX] Decide where the "+ Add display" links should be and be consistent throughout the admin UI.

[klonos]

...having filed #2342 and while writing this comment made me thing of the following:

• In the current design (the PR for Add the "Entity view mode" module to core #315), the "+ Add display" link is placed in the top-left in order to match the rest of the "+ Add xyz" links throughout the admin UI (content types, menus, layouts, etc.).

• The add entity display UI (that we are aiming to introduce in Add the "Entity view mode" module to core #315) is more similar to how the Views UI works though (things added via the "add" link are created as secondary horizontal tabs instead of a vertical table/list) and the "Add" link there is on the right of the displays.

So, I guess it all boils down to this:

1. Do we allow the Views UI and the Manage displays UI to be exceptions and have their "add" links to the right of the displays tabs?

2. If not, we should also (at some point) redesign the Views UI and move the "add" link to the left (and add a "+" to it)

3. If we do allow the Views UI and the Manage displays UI to have the "add" links to the right of the displays, perhaps we should consider making them look like "placeholder" display tabs instead of having them be rendered as links. Something like this perhaps: