Dragging a terminal group tab to the empty area should always move it to the bottom

[Tyriar]

Repro:

1. Create 2 terminals
2. Run git log in one of them so you can tell them apart
3. Drag the top terminal to the empty area, it's not moved to the bottom

[Tyriar]

Had a chat with @meganrogge and we're going to only rearrange the terminal group when dragging to the empty area. Unsplit will only be possible via the command and context menu.