Reworked icons section

* rewrote icon object
* rewrote src member
* rewrote type member
* rewrote sizes member

tidy
Non-normative changes to 4. Manifest section (Kenneth + Anssi)
(rebased on top of PR #139)
Merge pull request #140 from anssiko/manifest_section

Non-normative changes to 4. Manifest section (Kenneth + Anssi) … (rebased on top of PR #139)