Expose sensors to workers

[tobie]

Expose sensors to workers for perf reasons (avoid blocking main thread).

Actions:

• Figure out the permissioning story.

Certain use cases require sensors to have background access.
Using a more complex {{PermissionDescriptor}}.
(e.g. with a boolean allowBackgroundUsage = false; [=dictionary member=]),
might be the solution to relax this restriction.

• Amend IDL

[tobie]

Splitting that issue up into workers here and service workers in #106.