webassembly: resource management

Applications will have several options for how to handle resources. One of the decisions to be made is if the application should keep existing cross-platform resource handling code, or handle web deployment as a special case. Some of the options are:

1. Download using QNetworkAccessManager, from the (origin) server. This can be done incrementally, after the application has started, but may require changing application logic.

1. Populate the local (in-memory) file system before application start. Application logic that loads resources from the file system can then be kept-as is. The application to provides a list of resource file names to qtloader.js.

1. RCC. Normally RCC embeds all resources in the application binary, and this will also work here. It may be desierable to handle this differently: we don’t want to run all resources through the webassembly compiler, and also want to enable parallel and incremental resource downloads and caching. Instead, a new backend could be added where rcc will copy the resource files to the build dir and also output a manifest file that can be passed to qtloader.js.