Generalize vector feature selection

- Buildings, non-area polygons and linestrings can now be selected
- Cursor shapes adjusts to pointing hand when features below cursor
  can be selected
- Add new, fast hasFeatureAt method as an alternative to whichFeatureAt
  used to decide which cursor shape to use
- Move caching of items from GeoGraphicsScene to GeometryLayer. Avoids
  repeated sort calls and allows easier traversal of items in paint order
- Reduce GeoGraphicsScene::items() call locations to one (in
  GeometryLayer::render() and cached)

Maniphest Task: https://phabricator.kde.org/T4082