Add metadata for data inputs

Summary: Add the possibility to add metadata to DataInputs

Description:

Usually the DataInput of a Qt3dStudio scene needs to be connected to some other data provider inside the application.

In order to automate this process, it would be very handy to have the ability to add additional metadata to it.

See following example:

The designer creates a speedometer with a speed DataInput. The data provider is in this case some library which provided the speed sensor data from the car.

Now the goal is to automatically connected this at runtime.

Therefore an additional metadata in the DataInput can work as "key" to find the correct speed sensor data.

e.g.

DataInput

Name: Speed

Input: Float

NEW:

Metadata: "SensorName" => "package1.subpackage2.subsubpackage3.speed"

Preferable, the metadata is a "key" to "value" mapping. With QVariant as data type.

So I suppose to extend the existing class: Q3DSDataInput

with following method:

QVariant getMetadata(const QVariant& key);

and maybe also a setter:

void setMetadata(const QVariant& key, const QVariant& value);