TreeView: Add drag+drop support

[njannink]

Description

Closes #6323

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)

Checklist:

• The PR is submitted to the correct branch (master for dev, rel-1.x for maintenance).
• My code follows the code style of this project.
• I've added relevant tests.

[stsrki]

API-wise looks fine. Still need to run it.

[stsrki]

I tested it. Overall, it works fine.

I noticed one issue. If you drag a node into a childless node, the target node now has child nodes. But then, if you move that same node from its parent, it still shows the expand icon even though no child nodes are available. Refreshing the tree hides the expand icon.

[njannink]

yes I saw it aswell still working on that. There are still some issues with refresh and ObservableCollections it seems.

[njannink]

Moved dragdrop logic to a seperate helper class and resolved all comments.