GUI: rename methods returning item

Methods that return an item now contain "Item" in their names. Other methods working with the same item may not have this, for example

InstrumentItem* instrumentItem();

but

void setInstrument(InstrumentItem* item);

Methods returning SelectionProperty also have appropriate ending:

SelectionProperty<FormFactorItemCatalog>& outerShapeSelection();

This resolves #372 (closed)