Design system support in QDS

Add support to define and import design systems.

A design system support shall add the following components in QDS:

1. Style object for a Text style that defines the common styleable properties.
2. A color palette. 
3. QML module with singleton to access the style objects.

The Qt-Bridge plugins should export the style data whenever applicable.

The AssetImporter plugin in QDS should generate the Theme object and infrastructure and create QML bindings accordingly.