`MusicBrowseListPanel.qml` clear out business logic

`MusicBrowseListPanel.qml` and `MusicPlayQueuePanel.qml` contains logic to select the correct text. 

This applies to Delegate Item's imageSource, text and subText properties. The logic shall be either moved to a store or as these are methods attached to a delegate they could also move to a helper in the not yet existing "helpers" folder which should only contains JS files.

The logic behind this separation is that in the future we might want to move this code from JS to C++ for performance reasons. Also the UI logic shall be clean from business logic.