Don't show top "Open With" app for folders; only for files

Summary:
This patch returns the Open With items for folders to the way it was before D11569: with all entries shown in a sub-menu. This should make the context menu more relevant since most of the time when you right click on a folder, it's not to open it in some app.

The top app is still displayed inline for the app context menu--just not for folders.

Test Plan:
Top app Open With entries...

• Do not appear for local and remote folders and symlinks to folders
• Appear for local and remote files and symlinks to files

Folder:

File:

Note: screenshots reflect the Open With positioning from D11884: Move "Open" actions to the top of the context menu for files

Reviewers: Frameworks, elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: rkflx, elvisangelaccio, Zren, michaelh

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12206