client: Show when Remote Highlights unsupported

Add a header to "Remote Highlights" page to warn when the Quassel
core is too old to support core-side highlights, including a
"Details..." to explain the purpose and what version is required.
Core connection is still required to make the UI selectable,
following the convention set by other panels.

This should help avoid people asking about "Remote Highlights" when
their core is too old.

Wrap the rest of the settings page in a widget to simplify enabling
and disabling all of it.