Fix crash when an item is updated while we're in Calendar state (4 columns).

Fix crash when an item is updated while we're in Calendar state (4 columns).

layoutChanged is clearly not meant for the case where the number of columns
change. QSortFilterProxyModel asserts later on when handling dataChanged,
because it still thinks the source model has only 3 columns.

The fix isn't simple because EntityMimeTypeFilterModel gets column count and headers
from ETM's data rather than from its columnCount and headerData... and it doesn't
expect it to ever change.