GTK: Implement split drag and drop to reorder

[mitchellh]

This issue is for the GTK side of #10090. This also establishes a separate issue from #1525 which is being closed as a more generic issue. The user experience for dragging should be as similar as possible, within the reasonable constraints of GTK and idiomatic behavior of GTK apps.

[tarelda]

I would love ability to move existing tab into "side paneI". Is this even a possibility?