Documentation & fixes

Hello there.
Big update today since this release introduces a dogfooded documentation for the package:

Next to this here are the fixes:

• Iframe attributes have been added
• Fixed crash when using Javascript function New and Call with interface{}
• manifest.json has been renamed manifest.webmanifest in order to follow Google PWA guidelines

This release also introduces some experimental widgets that are layout related:

• Stack
• Flow
• Shell

Those widgets are used in the documentation introduced above. Since they are experimental, their API may change.