Split PO formatting preferences into Advanced tab

Move line endings and wrapping settings to a new Advanced tab. They are
rarely needed, never need by most people, and shouldn't clutter the main
preferences page.
Fix another ExplanationLabel-related sizing bug

For platforms where preferences are in a notebook, we need to call Fit()
on the dialog that contains it too.
Disable the Go menu in case of multiple selection

The functions there make no sense if there isn't single selection. For
some (Done and Next), doing anything in case of multiple selection would
be downright confusing.
Done and Next should unfuzzy

When the translator is "done" with an item, it should be marked as
final, not left in fuzzy state. Rationale: when hitting Cmd+Enter on a
fuzzy item, the intent arguably is to ack the fuzzy translation as good
and not requiring changes.
Put suggestion tooltip on the entire block, not just icon
Completely remove "modified" indicator from list

Don't show the green-dots sidebar for modified items. It wasn't obvious
to many people what the indicator meant and it wasn't terribly useful
(unlike e.g. in a programming text editor). Better to avoid the
confusion.