Add sceleton of GadgetCatalog and basic test for it.
Fix API inconsistency between addNewTabGadget and addGadget.
Do use "data-gadget-source" / "data-gadget-handler"  for all gadgets not
just for inline ones (without an URL).
add jquery-json that is now required istead of jstorage.
rewrite require-renderjs.js so that it respects dependencies and also loads required jquery-json.
Initial implementation of GadgetCatalog which can handle:
- multiple gadget repositories
- multiple gadgets per repository
- API to select proper gadget based on what it provides.
Use JIO's webdav storage to access GadgetCatalog index information.
Add needed basic jio libraries and adjust test accordingly.
Merge branch 'GadgetCatalog'