Provide fetch interface + wrapper component

[whoozle]

Things to consider:

1. #103 /_context.fetch for adding basic static api without necessity of having a component.
2. backend.fetch implementation, compatible with fetch with backward compatible xhr callback
3. bring core.Resource component back from the dead?
4. …
5. PROFIT

[comrat]

3. It's in 'controls' now but I can bring it to 'qmlcore/core' and implement its backend for html

[whoozle]

we have to bring up all committee and pull things together first. please do, and ask @vkrv for fetch implementation, he did it for appletv