Fix sort order of frames with simple naming scheme

QCollator::isNumerical works in platform-specific and Qt-version
specific ways, and cannot be guaranteed to sort mixed alphanumerical
strings in a numerical way. It just doesn't work that way.

This patch extracts all numbers for the comparant strings, and then
checks the last number in each string numerically. Now, if the user
would make frames like this:

scene1frame9
scene1frame10

Everything is fine.

frame9scene1
frame10scene1

will break; and unless we offer a kind of filename templating thing
where the user can specify which part of the filename provides the
frame number, there's nothing more we can do.

BUG:375885