Allow runtime layer/group access by name through specialized collections

[jng]

In Maestro 2.1, such code was possible

map.Layers["Parcels"]

or

map.Groups["Base Layer Group"]

This was due to the Layers and Groups properties being specialized collections.

In the move to 3.0, this pattern was no longer possible. Accessing layers by name now requires a full loop through the layers collection.

This ticket will reinstate the specialized layer/group collections making the above code pattern once again possible.

[jng]

Duplicate of #1852