qml: introduce background blur also to main view

In addition to the top bar and the playqueue, also use background blur in the main view.

Usually, applications make use of transparency throughout the UI. This is a deviation from the designs, but I think it looks better.

I made it with a stronger alpha (0.85) as opposed to (0.7), so the effect is not as strong as in the other areas.

Request review @chub.