Support installing PWAs

[BlobCodes]

Is your feature request related to a problem? Please describe. More and more applications are becoming Progressive Web Apps, which most browsers can install. I use some of those apps often, so support for installing them would be nice.

Describe the solution you'd like Full PWA support.

Describe alternatives you've considered None.

Additional context None.

[danimesq]

Part of: https://github.com/wexond/desktop/issues/147

[BlobCodes]

Part of: #147

I disagree with this being part of the packages API. PWAs are a web standard supported by all modern browsers, not a theme or some custom component.