[RFC] `SeekBar` widget

[nununoisy]

This proposal seeks to add a seek bar widget for controlling media playback.

A seek bar is very similar to a slider, but with a few key differences:

• A seek bar works with duration and not float values.
• A seek bar's minimum value is always zero.
• A seek bar's orientation is always horizontal.
• A seek bar optionally has a separate track region for displaying the buffer health.

Here is the proposed interface for a SeekBar:

• Properties
  • enabled: (in bool) Seek bar is enabled. Default: true.
  • has-focus: (out bool) Seek bar has focus.
  • value: (in-out duration) Current seek position. Default: 0s.
  • buffer-duration (in duration) Current buffer duration. Default: 0s.
  • total-duration: (in duration) Total content duration, analogous to Slider.maximum. Default: 0s.
• Callbacks
  • scrubbed(duration): User is scrubbing content, analogous to Slider::changed(float).
  • released(duration): User released seek bar after scrubbing.

The SeekBar implementations can likely wrap SliderBase since the two share so much functionality.