setHeaderGroup() changes the column count and filterAcceptsColumn()

but it didn't tell the QSFPM base class, which still had the old
column count in it.value()->source_columns, so index() for the
new columns would fail.

Caught by installing a QAbstractItemModelTester in EntityMimeTypeFilterModel
and starting korganizer.

(cherry picked from commit 571a93fc)
(cherry picked from commit a15e7f57)