BUG: Fix logic error in qtCheckItemComboBox and qtModelEntityItem

In order to be able to pick multiple items in comboBox, CMB uses a QStandardItemModel in comboBox. But CMB does not update the model when comboBox inits. Now CMB would initialize the model every time and disable the pickability of the entity summary.

When entities have been expunged, qtModelEntityItem should also expunge the corresponding items. A logic is added to address this issue.