Added Property::Key type to properly represent keys in maps

Added Property::Key type to properly represent keys in maps

Currently, Property::Map can have either string or integer
keys. The two methods GetPair and GetKey only allow iteration
over string keys.

Have added a single Key type that enables the retrieval of all
keys of Property::Map via the two new methods GetKeyAt and GetKeyValue

Have un-deprecated GetValue, which is still valid.

Added a new method Handle::GetPropertyIndex( Property::Key ), which
allows a lookup of a property from a property map key.

This is useful to enable property maps to be used as a property
aggregator when setting values on objects.

Have updated Scripting and PropertyBuffer to use new Property::Map
interface rather than the deprecated interface.

Change-Id: Ie2db5355fd661ed73a6f92b720d92ca1e6ab8d76
Signed-off-by: David Steele <david.steele@samsung.com>