Site Editor: Update preview frame dynamically when settings change

[jameskoster]

If you browse to a header/footer template part in the site editor and re-order menu items the preview updates dynamically:

nav.mp4

It would be nice to apply this treatment to other configurable items that appear in the details panel. One example is the "Posts per page" setting in the Home / Index template details panel. Currently the preview is only updated when you save changes:

posts-per-page.mp4

cc @ntsekouras

[jameskoster]

Just to note: this is somewhat related to #50392, where the intention is to include inputs for things like the Site Title in the template part details panel when appropriate:

So the solution to the "posts per page" issue will have similar applications elsewhere.

[youknowriad]

This seems to work in my testing (the post per page moved to the document inspector though)