Don't recurse into subdirectories when indexing ~/Downloads

We can't really make assumptions about the content of folders in the 'Downloads' directory. People may download any kind of content from the internet and then extract it "in place" in the Downloads directory.

This change was triggered by tracker#95 (closed) where several users reported system lockups caused by Tracker scanning video game data. The goal of this change is not to avoid the lockups, but to avoid scanning game data directories in the first place. Games often contain lots of audio, video, images and text files, which can end up in unwanted places like Gnome Photos and Gnome Music if we do scan it with Tracker.

This commit changes the default settings so that we index files in ~/Downloads but not directories. Hopefully this gives the right balance between: providing search results for files the user just downloaded, but not mixing up video game data with actual user content, and not consuming lots of resources unnecessarily.