retain selection after being dropped

[swaroopjo]

Describe the bug
After dropping the items, the aria-grabbed is set to false. In cases where i want to drag the same items into two different zone, I will have to re select the items and then drop to different dropzone.

To Reproduce
go to the demot: https://vivify-ideas.github.io/vue-draggable/example/

1. select one or more items,
2. drop them into different container,
3. the selection is cleared.

Expected behavior
The selection should not be cleared.

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Additional context
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