[recentdocuments:/] Filter out files that can't be browsed with a file manager

Summary:
This is a more appropriate version of D13797: Only add actual documents to Recent Documents. Basically we filter out anything that can't be browsed with a file manager, such as web linkns, appstream:// urls, etc.

This ensures that the contents of the recentdocuments:/ ioslave is relevant to the user.

Test Plan: Opened a link from Spectacle in my web browser. Without the patch, it shows up in recentdocuments:/. With the patch, it does not.

Reviewers: broulik, Dolphin, Frameworks

Subscribers: elvisangelaccio, markg

Differential Revision: https://phabricator.kde.org/D14893