Make the "is hidden due to non-collapsed selection" use the regular
caret-hiding mechanism, so that we make sure nsCaret and paint are
consistent on their caret visibility.
As part of this simplification:
* Remove LookAndFeel::IntID::ShowCaretDuringSelection, it's 0 on all
platforms.
* Remove nsCaret::IsMenuPopupHidingCaret. Is really broken as per the
comments (it assumes single-process mode, doesn't deal with shadow
dom, and it's not like it's particularly useful anyways since the
menu popup could be semi-transparent or what not).
Differential Revision: https://phabricator.services.mozilla.com/D205557
