GUI: replace member variable that points to an InstrumentItem by pointer to a subclass thereof
Class BackgroundForm
has a member m_instrument
that points to InstrumentItem
while actually only a pointer to m_instrument->backgroundItem()
is needed.
The same applies to a number of other sub-editors: their m_instrument
member should be narrowed down to what is actually needed.
This helps to decouple classes and reduce include dependencies.
Edited by Wuttke, Joachim