Change default cover image

Why?
* The existing image is just a placeholder used hundreds of times elsewhere, with no source, bad resolution and uncleared copyright

Why it looks so...boring?
* Generic, distinguishable image
* Semi transparent image that works with dark and bright backgrounds, without dominating the skins design.
* Does not replicate Spinnys, and does not re-use the iTunes like default cover image used elsewhere. 
* Png hdpi ready

If this is getting merged, i'll add the source to ``res/images/templates/ic_template_library_and_preferences.svg`` (Public domain license).