TreeViewFind: Optimize speed by not creating a QTextDocument per index

Change-Id: I74027b051d85698d95f92cb212182281013d256c
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
TreeViewFind: Fix endless loop if tree view has nothing selected

If the tree view has nothing selected, the search would wrap endlessly
because the current index is invalid, but wrapping restarted at the
first toplevel index.

Change-Id: I31badb2038c8752d2c91e6fcaff69138c846383e
Reviewed-by: Daniel Teske <daniel.teske@digia.com>