Obsolete because of removing GUI <-> domain translations in the last commits

Get rid of `GetVectorItem` function in the favor of direct VectorItem access. Later will be changed again to direct kvector_t access methods in (BeamItem, DetectorItem, and other classes).

Redmine: #2051

1. Mask container can now be aquired directly from RealDataItem
2. importMasks and clearMasks methods become virtual in
   InstrumentItems