multimedia loading saving offsets fixed

[jackft]

Purpose of this PR

1. media file offsets were not saved in the eaf/mmeaf files properly
2. the abstraction of a primary media file caused issues
3. media file names were not preserved when loading media in the web app.
4. we weren't able to apply template files to existing documents, e.g. add new types to do new annotation.

what changed?

1. now, we save media files in eaf/mmeaf with offsets while trying to preserve their original file path
2. now, there is one media dialog. if a user loads a .eaf file in the web app, they can then see what the name of the file they are looking for is called
3. now, there is an "apply template" function, in the file menu. this allows the user to select a template file from which they can copy types & tiers into the current annotation project.