Media player UI cleanup

• Use same design on lock screen than in settings
• Use same width than notifications intead of being somewhat smaller
• Use same border radius than notifications
• Move closer to #543

Closes: #370 (closed), #371 (closed)

Before:

After:

This is moslty meant to make #543 easier. If that looks good I'll merge and look into the vertical mode for the lockscreen and the progress bar.

/cc @devrtz