[Extensions] Keep the IDL file close to the implementation

Teach the build system to process every file in the source section
ending with .idl. Now the .idl files can stay in the same directory
as the implementation making it more self contained.
[Extensions] Move extensions tests to the new IDL rule

The buildsystem now supports processing of .idl on the source
section, there is no need to include the IDL processor again
for the browser tests.
[Dialog] Use a more descriptive name for parameters

We have no plans to generate the JS shim in a short term, so use a more
verbose description of the parameters matching the IDL.
Merge pull request #882 from tmpsantos/jsapi

[Extensions] Keep the IDL file close to the implementation