Use setShowOpenOrExecuteDialog(true) with OpenUrlJob

It makes more sense for a file manager to always ask the user:
 - whether to execute binaries
 - whether to open an executable script as text or execute it

The openOrExecute dialog from KIOWidgets has a "don't ask again" checkbox,
and that setting is also accessible from Dolphin's settings dialog.

See the discussion at https://bugs.kde.org/show_bug.cgi?id=429839#c10