Merge Slider and Linear Progress Functionality

[wwwizzarrdry]

The QMediaPlayer extension is great, but I’m rolling my own music player with Howler.

I just spent a lot of time making a time rail because the functionality I require is half inside the Linear progress and half inside the Slider components.

A couple suggestions:

1. Update the Slider to be able to adjust the thickness of the rail.
2. Update the Linear progress component to be interactive like the Slider input. I’d really like to be able to touch a point on the progress bar and have it return the percent/value of that position.
3. Update the linear progress bar to enable a floating label like the Slider has.

[nothingismagick]

Please post this to the github repo.