B2Qt Demo update - Media Player

Update Media Player demo to comply with Design Guidelines for Demo apps that can be found from [http://brand.qt.io/demos/]. At minimum this means:

• Change the color schema to follow the dark background and color template in the guidelines
• Add [i] Info button page and describe two to five main points why this demo exists. The intent of this page is for people looking at the demo on their own to understand why this particular demo is here.