Handle file types in document registry

[blink1073]

Fixes #2594.

Adds better utilization of IFileType across the application.

• Used by widget factories instead of raw file extensions
• Used by mime extensions to establish new file types
• Used by the file listing to choose a file icon
• Adds support for pattern in IFileType for matching file names

Also cleans up the handling of the default text model factory for the registry.

[blink1073]

cc @cameronoelsen @ellisonbg

[afshin]

Awesome, thanks! Now we can support weird file types like Makefile, Rakefile, Dockerfile, etc., that do not have file extensions.

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Yep, after this PR a Dockerfile is opened with the right syntax highlighting automatically.