Fix ToC action when cursor is in adv. F&R pane

Fix ToC action when cursor is in adv. F&R pane

This important part is the last point, the rest is what is needed to
make it happen.

* implement (FindAndReplace|FindAndReplaceWidget)::hasWorkArea, that
  tell whether a work area is own by the advanced find & replace
  widget.

* factor out method find() from GuiView::findOrBuild.

* implement GuiView::hasVisibleWorkArea, that tells whether a workarea
  is visible in the view (current tab in a split or adv. f&r
  workarea).

* Finally, in TocWidget::sendDispatch, change the current workarea
  temporarily to the document workarea before dispatching the
  function. The code tries to be as careful as possible to handle all
  cases. The future will tell whether it is good enough.