Added ImGuiTreeNodeFlags_SpanAllAvailWidth flag option to TreeNodeEx

[melix99]

Fixes #1897

[ocornut]

FYI, the reason I haven't merged this yet is that I am considering a reoverhault of the overlapping system to support top-to-bottom overlap by default, which would make this flag unnecessary.
Working on this.

[ocornut]

Cross posted #1897, #2438, #2451

• Added ImGuiTreeNodeFlags_SpanAvailWidth which does what is suggested in Extra hovering region for TreeNode  #1897 and implemented in Added ImGuiTreeNodeFlags_SpanAllAvailWidth flag option to TreeNodeEx #2438, Added ImGuiTreeNodeFlags_UseFullWidth to allow using the full content width for the interaction area for unframed tree nodes. #2451, it extend the hit-box to the right-most edge of the working area.

• The reason I have been stubbornly defering this for a while is that I would like this behavior to be the default but that will requite a refactor of the hit-testing/overlap system toward front-to-back hit detection, which is not a trivial task and always gets delayed. I was wrong to delay this, because whenever the day happens that the hit-system works front-to-back, we'll simply be able to redefine ImGuiTreeNodeFlags_SpanAvailWidth to zero and there won't be any API breakage from user's point of view.

• Added ImGuiTreeNodeFlags_SpanFullWidth flag. This is a little subtle, but this one also extend the framed/highlighted area to the left-most edge of the working area, while keeping the arrow/text in the same spot.

if (ImGui::TreeNodeEx("TreeNode", ImGuiTreeNodeFlags_SpanFullWidth))
{
    if (ImGui::TreeNodeEx("TreeNode", ImGuiTreeNodeFlags_SpanFullWidth))
    {
        ImGui::TreePop();
    }
    ImGui::TreePop();
}

Sorry for the delay!

[PathogenDavid]

Thanks, Omar!