Make highlight-on-hover a user preference.

This also

• makes highlighting more robust against unreported (i.e., buggy Qt) "leave" events (by also resetting the hover selection when the mouse leaves the resource panel dock widget).
• draws a background on Qt items as the mouse hovers over them (when highlight-on-hover is set) so that users get feedback on when Qt has skipped events.
• fixes transforms applied to the representation; previously, transforms were not applied to selected/highlighted components.
• fixes the build against latest ParaView master. The old WRAP_EXCLUDE and WRAP_EXCLUDE_PYTHON CMake source properties are no longer used; classes must pay attention to the __VTK_WRAP__ macro to exclude themselves if needed.
• adds user preferences for selection/hover colors and a default model-face color (for model edges, we should use ParaView's existing edge color). Thanks to Utkarsh for explaining the <Extension ...> element in PV's SM XML.
• fixes the "can't deselect" bug (the SMTK model-representation's actors/mappers were not being updated when no entities were selected).