reload metadata

[jonorthwash]

A little refresh button somewhere up in the "Recording" section that does the following:

• Rescans files and updates anything in metadata as needed
• Does not erase traces! Maybe traces and metadata should be separated into different files?
• Updates the interface to include any new files (or remove ones that no longer exist)

[jonorthwash]

This will involve separating out the "scan project" code into a separate function, and having both the startup code and the "reload metadata" / "rescan project" button (I think I like the latter name) call it.

I believe the "scan project" code is approximately this section: https://github.com/SwatPhonLab/UltraTrace/blob/38b43b40311e8061adac4eb79365e6c626faf04b/ultratrace/modules/metadata.py#L64-L150)

The definition of MIMEs should be moved to outside of this function, as a static variable. Note also that the MIME stuff will interact with #179.