No interaction for now.

GadgetIndex representation.
Adding a new tab should rmove existing gadget and its representation.

render gadget immediately.

so they don't become orphans.
TabbulkarGadget keeps track of most recent gadget being used and cleans
it up when new one is added, thus reducing trash in Gadgetindex and DOM.

existing Gadget methods.

call them from gadgets without taking care for real implementation
inside. Allow force rebind for InteractionGadget.
Extend test for mentioned changes.

Add multiple instances test for getSelfGadget.

Allow getSelfGadget to be used in handler function.
Allow to define multiple levels of urls.

Recreate default route using proper RouteGadget.

between different applications.

Pass gadget_id to route handler function so it can pick up gadget
instance easily.
Extend test.