Context view rendering DPI improvements

Current track nothing playing: Handle different applet heights better, render svgs with high DPI, use constant rendering resolution to avoid re-rendering during scaling, use unique render key names to avoid funny svg mixups when both have the same size. Increase rendering resolution for default "no cover" image. Port AppletHeader to QtControls2, increase icon DPI there, too. Also fix dragged applet toolbar item size not restoring to initial after releasing drag.