[ui][diagrams] Fix drag and drop to reorder stacked diagram not working

[nirvn]

Description

This PR fixes #59505 whereas the stacked diagrams tree view was not allowing for drag and drop re-ordering of sub-diagrams. It's an important UI/UX that users would definitively expect here as we allow for drag and drop re-ordering for symbology renderers and rule-based label renderer widgets.

In addition, while fixing this, I've harmonized the tree view by removing the "Enabled" column in favor of adding a checkbox in the first column where the diagram name is located (like we do pretty much everywhere else), as well as adding a nice touch of night mapping (eye toggles!).

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit 0b09072)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit 0b09072)

[DelazJ]

Thanks @nirvn
One point raised in the issue report I can't tell if covered here is the handling of sub-diagram placement settings. Per the initial PR:

Note there are some shared settings like placement settings (e.g., over point), that the stacked diagram will take from its first subdiagram. Thus, other subdiagrams won't be able to set or change those specific settings, unless they become the first subdiagram in the stacked diagram. Therefore, when editing a subdiagram that is not located in the first place, we show labels to let users know why they don't see the expected widgets (e.g., placement ones) and where they can find them (i.e., in the first subdiagram).

Some options of subdiagrams not topmost are currently greyed out. My suggestion was to allow any diagram options to be modified anytime, regardless the rank of the subdiagram, and then have a method that would apply only placement options from the topmost to the whole stack. That way, a simple-drag-and-drop would be enough to preview what I expect, and not require subsequent placement options modifications. And if I switch again the order...?
Better have the subdiagram options set when I'm configuring the subdiagram, once for all.

[nirvn]

@DelazJ , let's merge this PR which gives us generic re-ordering support here and deal with the extra UX enhancements you mentioned in subsequent PRs if that's OK with you.