With help from Palid.

This does two major things:
1. Makes the tile part of a file body clickable to trigger a download.
2. Refactors a lot of the recyclable code out of the DownloadActionButton in favour of a utility. It's not a perfect refactoring, but it sets the stage for future work in the area (if needed).

The refactoring still has a heavy reliance on being supplied an iframe, but simplifies the DownloadActionButton and a hair of the MFileBody download code. In future, we'd probably want to make the iframe completely managed by the FileDownloader rather than have it only sometimes manage a hidden iframe.

Fixes element-hq/element-web#18239

Turns out the tooltip component doesn't copy over class names.