Slider: Add bottom and top ticks and tick offset

[beicause]

Fixes #103839.

Before:

Edit: This PR adds tick_position and tick_offset to allows the ticks to be placed at the top, bottom, or center with custom offset.

[Calinou]

This will affect existing projects (albeit not in major ways), so it would be best to make it configurable with a Tick Offset theme constant.

[AThousandShips]

The offset default should be such that it preserves the original behaviour, and allow you to change it to compensate, or this will still break compatibility

[beicause]

If we accept tick is drawn at the center, the original behavior still can't be guaranteed by the default value of tick offset, because the widget_width (the size of the slider stylebox) may be modified by users.

[bruvzg]

Both before and after are pretty strange UI choices, sliders usually have either no ticks and round grabber, or offsets ticks on the one or both sides and arrow/rectangular grabber.

It might be worth adding separate top/bottom ticks and offset and length theme constants for both to allow full customization.

[beicause]

I don't think top bottom length are necessary. To customize the size&color of ticks, the simplest way is to use GradientTexture2D as the icon.

[KoBeWi]

For reference, here's how ticks appear in Windows dialogs:

And in Chromium:

If we accept tick is drawn at the center, the original behavior still can't be guaranteed by the default value of tick offset, because the widget_width (the size of the slider stylebox) may be modified by users.

You can at least guarantee it for default theme.

[beicause]

After consideration, I think adding top and bottom ticks makes sense, but the alignment should be changed to the upper parts of slider for top tick and lower parts for bottom tick.
(now maybe I should change the title of the pr :)

[KoBeWi]

Is there any use in having style for both top and bottom tick? All examples of double ticks I found are symmetrical, so you can just repeat the tick icon (at most it could be mirrored maybe?). Likewise, the offset can be single value for both sides. There could be a property that controls whether bottom, top or both ticks appear.

[Charpurrr]

personally id still really like a way to have them run through the center, just for the customizability (and stylisation) option. could integrating the original pr as a toggle or similar be considered? :3

[beicause]

It's OK to add a "center" option. And we can just keep one tick_offset instead of having a separate tick_offset for "top", "bottom" and "center" then.

[KoBeWi]

Looks good. Compatibility breakage could be avoided with proper defaults, but not sure if it's worth it; it would require some awkward values. The new ticks look better IMO and they are similar enough.

[beicause]

For the top or left ticks, I used  draw_polygon  instead to make them symmetrical with the other side.

[KoBeWi]

You can use draw_rect() instead and provide rectangle with negative size. It mirrors the texture.

[beicause]

Added TICK_POSITION_ prefix to the enum to follow naming style.

[bruvzg]

Seems to be working as expected.

[akien-mga]

Thanks!