Move style definition into template file

 - The style of an element should be completely
   defined in the template file.
Improve visual of the CoverToolbar

 - Convert from ToggleToolButton to ToggleButton.
   ToggleButtons have 3d outline, which improves
   the visuals a lot.
 - Remove style definition for inline toolbar as it
   is not needed anymore.
Always initialize CoverStage

 - Initialize the CoverStage if the application
   starts up, to get a valid cover even if no
   cover could be loaded but the service supports
   integration.