csharp: Property Indexer implementation

csharp: Property Indexer implementation

Use Indexers to use brackets, eg [i], syntax.

Keys now can be used as following:

var someVal = obj.SomeProperty[key];
obj.SomeProperty[key] = someNewVal;

And for multiple keys:

var someVal = obj.SomeProperty[(key1, key2)];
obj.SomeProperty[(key1, key2)] = someNewVal;

T8384

Reviewed-by: WooHyun Jung <wh0705.jung@samsung.com>
Differential Revision: https://phab.enlightenment.org/D10791