client: Unify CoreHighlightSettingsPage tooltips

Move setting tooltips to a generic function, setupTableTooltips(), to
keep tooltips consistent.  This fixes outdated tooltips in the
highlight ignore rules table and reduces duplication.

Add another function getTableTooltip() for getting the tooltips
themselves.  Unfortunately, QTableWidgetItem does not inherit from
QWidget, so function overloading (or equivalent) is necessary.  Alas,
std::variant is C++17...