Control fixes

• The SRGTimeSlider borderColor property has been removed, as having a border color was never really useful (sliders with a border color were ultimately ugly). All tracks (elapsed time, remaining time and buffer) are now drawn equally thick and with pixel-perfect precision. The track thickness can be controlled with the new trackThickness property.
• The intrinsic content size of all controls is now correct.
• When entering picture in picture, the picture in picture button does not toggle erratically between states anymore.