Delete resource by index from model, instead of selection index

Deleting a resource with "Show deleted resources" disabled will update the proxy model and next index in selection now points at wrong item, collect KoResource pointers first, and then fetch its index from model instead collect resource ids first, and then fetch resource index from model by id instead.

BUG:439594

Test Plan

• Open Manage Resources window
• Switch to Gradients
• Make sure Show deleted resources is unchecked
• Select first three items
• Delete them
• Observe that first three items were deleted, instead of first, third and fifth items

Formalities Checklist

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