Basically, introduce a small check for this corner-case situation.
Setting a model doesn't just store a pointer, but also resets the current
index, changes the line edit completion model, and so on, and seems silly
to trigger changes in such cases.
[ChangeLog][QtWidgets][QComboBox] A QComboBox does not reset itself any
more when setting the same model more than once.
Change-Id: If028b36cdfaa5552c156dd900e123ca9a04d4e3d
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
95092e2011