ENH: update QT interface code for ParaView reader (issue #337)

- remove old (ParaView-3) files

- Works in 4.4.0, 5.0.1, 5.2.0 etc

STYLE:
- slots now use SM properties directly without a second lookup.
  This reduces exposure of the QT elements and simplifies the coding.

- avoid focus borders on the Qt elements

- place the "use Polyhedron" checkbox into a column

- move "Cache Mesh" down in the GUI (an advanced feature and thus
  should be less prominent)

- obtain button labels/tooltip directly from the XML content