qt: Don't display progress dialog as modal

Updating the dialog will call processEvents, which could end up
processing another dialog progress update, calling the event loop again,
and so on until we stack overflow.
Basically, having a modal progress dialog means that any user of that
API could potentially cause stack overflows if update calls were to be
unpaced.
Part #2 of fixing #18640 and #17060

(cherry picked from commit 55e6be0e)
Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>