Add SwTextBoxHelper::findShapes

It builds a textbox -> shape map, so methods interested to pick the
shape instead of a shape's textbox can call it and act accordingly if
their textbox is in the map.

Change-Id: I0f30d64a284eb461f462ed6c0a36c88271153f04
sw: update hintid comments to match reality

Change-Id: I9c7f620e08865531c70b1c4e655ed9f0aed9d319