Update Slider Element

[willpatera]

• Slider handle should be implemented using FitBox so that it could be appropriately resized if global UI scale implemented in the future.
• Rendering
  • label and value above slider element
  • Line for slider below label + value

[willpatera]

• Added line to the slider element.
• Moved the label and slider value above the sliding element. This makes the slider element 2x taller than other elements (we can change this if desired).
• Slider handle remains as a Vec2 - making it a separate FitBox as described in notes above might be overkill unless we need more control in the future.

Todo

• Implement the step slider marks & test.