Improved value type support in QML

Some more possibilities have been identified:

1) allow loaded modules to provide different value-types.

• there is currently limited support for this in QtQuick2, given that the engine will allow other value-type providers, but it still requires the metatypeid of the value type to be <256 due to the 8 bit typeId in the packed structure used during compilation (FIXED - in 4350877d6deb58f36df24164c6edde3302a3f1a3)
• also, the property syntax is supported via hardcoding in the parser (eg, "font", "color", "vector3d" etc) as opposed to dynamically via "valid property type strings" lookup after module loading completes)

2) use a non-qobject custom object which is per-instance rather than per-type to allow sub-property signal notification, etc.