Make video-browser a zapp

This first step towards #6 turns the video browser into a zapp. This
means that one can deploy it with zpm. The next step is to remove the
two helper zapps.
Remove helper zapps (fixes #6)

The helper zapps are no longer needed. Instead, the functionality has
been rolled into the main video browser zapp.

We must now do part of the job of 'zpm deploy' at runtime: we must add
the Swift path to the video-browser.zapp file to the extract-meta.json
and extract-thumbnail.json files. These JSON files were extracted from
the helper zapps before they were deleted.
Merge pull request #16 from mgeisler/single-zapp

Put everything into a single zapp