refactor: move slider seekranges & handle drawing into own functions

[Commander07]

This PR aims to reduce code duplication and complexity by separating slider seekrange & handle drawing from the rest of the slider. As a result seekrange drawing is now the same on both the normal and persistent seekbar which closes #173.

Changes

• Created draw_slider_handle function which takes an element its ass and draws a handle using settings provided by user_opts.
• Created draw_slider_seekranges function which takes an element its ass and draws seekranges using settings provided by user_opts.
• Changed both sliders and the persistent seekbar to use the new functions,

Preview

Persistent seekbar now respects user_opts.seekbar_cache_color

[Samillion]

Absolutely beautiful. I think at this point you've improved most of the seekrange behavior on ModernZ. Much appreciated.