Check QSortFilterProxyModel behavior

Assuming the following:

• In QAbstractItemModel, only one change is allowed to be in flight at a time, i.e. rowsAboutToBeInserted/rowsInserted must not nest or overlap.
• In one rowsAboutToBeInserted/rowsInserted change, only rows with a common parent can be inserted.
• When changing filters in QSortFilterProxyModel, rows on multiple levels, with different parents, are inserted at once.
  If all of these three are correct, I cannot see how changing the filters can be correctly implemented in QSortFilterProxyModel.

This item is about the following:

• Check that the three assumptions above are correct. Make sure that the documentation clearly states whether these assumptions are correct.
• Extend QAbstractItemModelTester to check consistency in number of rows/cols. I.e. rows are only added between rowsAboutToBeInserted and rowsInserted.
• Apply QAbstractItemModelTester in all tests of QSortFilterProxyModel
• Extend this to QFileSystemModel