[Folder View] Open selection in preferred application instead of running them

This makes it consistent with Dolphin which will run scripts only if one is selected.
If multiple are selected, they are opened in their preferred associated application (e.g. Kate) instead.

I also removed the restriction on opening folders when only one item is selected.

CCBUG: 375793

Differential Revision: https://phabricator.kde.org/D4368