Greatly reduce memory consumed by QQmlEngine instances

Due to a inefficiency in our InternalClass implementation,
we were not sharing the string and attribute table between
internal class instances.

This was extremely inefficient with the Qt object, as it created around
1000 internal classes with large string and property tables. With the
patch these tables are now shared.

Reduces memory consumption of a QQmlEngine instance from around 6.5M
to a couple of 100k.

Change-Id: Ib763f31deca0808c000ac2c30aa0b05e806bda40
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>