Add file icons to FileList module

[janbridley]

Description

The FileList module can now display file icons next to the filenames. This is (in my opinion) a nice feature to have, and opens up a solution for #73, which could use the icons as handles for the file preview popup or as a download button.

Before	After

Motivation and Context

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• The package documentation and framework documentation in signac-docs are up to date with these changes.
• I have updated the changelog and added any related issue and pull request numbers for future reference.

[bdice]

Maybe tighten this up a bit, since x- is pretty broad:

Suggested change

	if "x-" in mtype or "json" in mtype:
	if mtype.startswith("application/x-") or "json" in mtype:

[janbridley]

Actually, on further review, I'm a bit unsure how to handle this -- "x-" is quite broad, but both "application/x-" and "text/x-" are required to handle all common code cases, which ends up falling back to the same behavior as far as I know. We could use something like content-types, but that's an extra dependency. Let me know your thoughts

[bdice]

No big deal. We can go with "x-" in mtype if it works well for your needs. Let's avoid another dependency.