Implement the Tab.active property.

Allows developer to query if a tab is currently active.
The property is also writtable so when set to "true"
the tab becomes active. Setting it to any other value has no affect.

[TIMOB-12983]
Implement property TabGroup.activeTab.

[TIMOB-12983]
Fix a bug which caused proxy corruption with setters.

Do not override the TiObject* of a JavaScript proxy object.
This was leading to the proxy becoming un-usable since it
no longer pointed to a valid TiObject* instance.
Merge branch 'TIMOB-12983'