Fix file dialog usage in app and plasmoid

fileDialog.open() does *not* block, so we need to move the sending to onAccepted. Not sure why I thought this ever worked.

BUG: 423975
1 job for master in 14 minutes and 21 seconds (queued for 10 minutes and 49 seconds)
Status Job ID Name Coverage
  Build
passed #57929
linux kf5-qt5 qt5.12

00:14:21

0.0%