TocWidget: fix an erroneous collapse and optimise updates based on profiling

TocWidget: fix an erroneous collapse and optimise updates based on profiling

TocModels::reset() in GuiView::structureChanged() collapses the TocWidget, and
therefore requires an update right after, which was missing.

In fact, profiling TocWidget::updateView() shows that delaying the update is
good only for fast keypresses (essentially movement). It costs 5% of a
char-forward operation in a document with approx. 100 table of contents
items. The update optimisation has been rewritten to take this data into
account.

This also fixes #9826: Outline disclosure of subsection content disappears one
second after doubleclicking content item.