Ui::add_sized does not work as expected with Slider

[koalefant]

Ui::add_sized does not work as one would expect when Slider is passed in. Instead Spacing::slider_width is used for its width.

The expectation here is that slider can be resized by supplying size in add_sized.

[wucke13]

I guess this is tricky, because the slider can also have a drag value at the side, which changes width depending on the value from the slider. I guess the correct thing would be:

1. Get the maximum possible width of the dragvalue of the slider (with trying both the minimal and maximal value + suffix/prefix)
2. Make the dragvalue it's maximum width
3. Use the remaining width for the slider

[emilk]

The problem is that Slider is not an "atomic" widget, but a collection of smaller widgets. It also makes it impossible to center in a layout.

In this case it could perhaps be fixed by looking for and respecting the "justified" flag in the parent layout (which is set by the add_sized function).