Fix item positioning, text alignment & unwanted clipping of ItemList items

[davthedev]

Implements part of the fixes mentioned in #88217

The following points are fixed:

• Clipping of selection indicator at the top or bottom of the list
• Gap at the bottom of the list in rows mode when the end of the scrollbar is reached
• In grid display, labels were not properly centered, a little offset to the side was happening
• Separators overlapping some items by a pixel or two beyond the border
• Inconsistent scale of vertical separation, which is double in comparison to horizontal separation (if you put 10px vertically, it displays like 20px, while if you put 10px horizontally, it remains 10px). Default & editor themes adjusted to compensate.

For the sake of consistency, the horizontal separation which did nothing on list rows (non-grid) mode is now acting like an internal padding, the same way it does for grid items.

This is a rewrite of the positioning algorithm so that internal calculation of item positions now corresponds to the actual screen rendering.

In the base situation before this fix, items positions are calculated with a gap in mind then grown on the four sides to cover that gap. This is responsible of the clipped selection indicators.

With this fix applied, the position calculation takes into account an appropriate padding to give the same effect as in the initial situation, and positions them correctly to align with the container borders without unwanted clippings and paddings.

No more clipping at the top and left

No more extra gap at the bottom

Text is correctly centered within grid items

Row items can have horizontal padding, leveraging the h_separation parameter that was not taken into account in this mode

[Mickeon]

When making #88370 I encountered several minor nuisances that I feel like this PR addresses.
The code also looks cleaner and I feel like users would appreciate this, even if it would ever-so-slightly change how ItemLists look.

Still requires testing to make sure nothing silly is overlooked.

[Calinou]

Tested locally in the editor and control gallery demo project, it works as expected. However, in the default project theme used by the control gallery demo project, vertical spacing is now reduced, which makes it harder to click individual items:

Before	After

scene/resources/default_theme.cpp needs to be modified to increase ItemList's v_separation constant to get a similar appearance to before.

We should make sure the change in how ItemList's v_separation theme item is interpreted is clearly documented in the release notes though, as existing themes will need to double the value for the current appearance to be retained. As such, we should avoid cherry-picking this PR to 4.2.

[davthedev]

Indeed, I forgot to adjust the default theme after the vertical spacing fix. Will have a look.

I don't think some smaller item height makes the thing harder to click, on the opposite it becomes less small-screen-friendly and requires more work when using the default theme in a low-res, scaled-up game. But I agree with your point that we should avoid breaking existing projects appearance as much as possible and limit those to more important version upgrades.

I have additional fixes and improvements that I plan to build on top of this work, btw.

[davthedev]

I limited the blast radius of this refactoring so that we can merge it in 4.2. Perhaps I can revert the v_separation to use the old 2x behavior to avoid breaking current themes. What do you think?

Saying this because I have other points to refactor that can break existing themes, and I want to deliver those in a different PR that would wait for the correct timing for such changes. This PR would have not point then as it is considered breaking in the current state.

[akien-mga]

Thanks!