[Controls] TimelineSlider: Improve slider's behaviour

[cbentejac]

Description

Building up on top of #3059, this PR improves two elements of the Sequence Player's timeline slider:

• The slider's position could be misaligned with the frame number markers. For example, there could be a marker for frame n°5, the slider would be set for frame n°5, but the slider would not be visually aligned with the marker for that frame.
• The last position the slider could reach was the far-end of the timeline, which we do not want. Instead, we want this space to be reserved for a phantom-frame, and the slider to be physically blocked right before reaching this phantom-frame.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.56%. Comparing base (e270ef5) to head (16a5756).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3094   +/-   ##
========================================
  Coverage    83.56%   83.56%           
========================================
  Files           81       81           
  Lines        10322    10322           
========================================
  Hits          8626     8626           
  Misses        1696     1696           

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