When the source models don't have the same number of columns, the proxy
keeps only the smallest number of columns across all source models.
Afterwards, if a source model emits dataChanged in a column past
that number (a "hidden" column), the proxy needs to ignore it rather than
assert.
But also, if the source model emits a dataChanged signal across both
visible and hidden columns, then the last column number needs to be
adjusted so that the signal is correctly processed and forwarded.
Task-number: QTBUG-91253
Pick-to: 6.1 6.0 5.15
Change-Id: I939e8ec0faf41370472f86785851292e4372f72c
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
f6efbd23b5