Fix drag and drop for stacked diagrams (follow-up)

[gacarrillor]

Follow-up #60278.

This PR adjusts issues and improves consistency with drag and drop behavior in other parts of QGIS. See #60596 for screencasts of the buggy behavior in the current master.

Namely, this PR:

1. Replaces the drop indicator, switching from a box to a horizontal line (which expands to the whole row). The rationale is that stacked diagrams don't handle (yet) a parent-child hierarchy (i.e., one cannot have nested stacked diagrams in the GUI), but act instead as a flat list. This is consistent with drag and drop for other flat lists in QGIS (e.g., classification symbol lists or graduated symbol lists).

2. Fixes an issue that moves the dropped item to the end of the list (i.e., an append operation), even if the desired position was, e.g., between other items.

3. Fixes an issue that makes it possible to obtain copies of the dragged object.

4. Enables moving multiple items and positioning them in the expected way (e.g., if a diagram A is below a diagram B before the drag, it should continue being below B once dropped).

5. Minor fixes: Allow dropping items below all other items (i.e., in the blank area), as well as in the whole row span (i.e., also for columns > 0).

6. Clear leftover current index in the view after the drag'n'drop. This is how other flat lists behave, e.g., Graduated and Classification symbology for vector layers.

Screencast of the result:

dragndrop_stackeddiagrams.mp4

Fix #60596

[github-actions]

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Debug symbols for this build are available here.
(Built from commit 5faf094)

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(Built from commit 5faf094)

[gacarrillor]

(Rebased)

[gacarrillor]

Thanks for the review @nirvn, I've addressed the comments to keep the fixes while keeping copy&paste eventual addition still possible (which, by the way I'd be happy to add in another PR).

[gacarrillor]

The last commit clears the current item from the view. As you can see in this screencast, the current item is a leftover, since it sticks to the dragged item's index, showing no valuable information to users.

dragndrop_stackeddiagrams_active_items.mp4

Additionally, this clearing brings some harmonization with other flat lists after a drag'n'drop, e.g., Graduated and Classification ones for vector layer symbology.

[gacarrillor]

(Rebased to build on top of working CI)