Inspector: Consider pill button to split between Block and Post/Page settings

[jasmussen]

As discussed in #1356 (CC: @GaryJones), and also per recent discussions on #1716 and #1251, there's some confusion around how to access advanced block settings.

Let's explore renaming the sidebar from "Post Settings" to "Settings", and add a pill button switch to the sidebar itself. The pill button would have two options, "Post Settings", and "Block Settings".

This would make the cog behavior more obvious. It would also let you see just the post settings, without having the block settings interfere, as you select blocks.

This pattern mimics how Keynote does it.

[jasmussen]

Here's yet another mockup:

[GaryJones]

As that last mockup is, it doesn't leave much space for longer translations, or plugin customisations (i.e. wanting to add a third "tab").

[jasmussen]

Could we do with "Post" and "Block" as the titles? Also, I wouldn't imagine this area be pluggable.