HTML5 progress bar fixes (gzip+engine load) (defold#5211)

* Load engine and start updating progress immediately

Still missing progress when loading the .wasm file though

* Update dmloader.js

* Added unified file downloader

* Fixed indentation

* Cleanup

* Indentation fixes

* Update dmloader.js

* Improved file downloader. Added progress for wasm download.

* Reverted name change for progress update function

* Update dmloader.js

* Removed use of spread arg

* Use HTTP HEAD request to get resource size

* Add estimated size to file download requests

Use estimate when no size can be computed (gzipped content)

* Made wasm and asmjs size configurable for the progressbar