Improve tabs in Terminal view #3015

• Make the Terminal View a composite E4 view that contains a part stack.
• Adding a new tab would then simply mean adding a part to the model's part stack.

This would automatically inherit the features and behavior we already have (drag and drop, detaching, minimize/maximize, persistence, etc.).
It would also let others extend and contribute through an E4 model fragment.
For example, one could add a static, non-closable terminal part, or contribute an action that depends on the terminal type.

Originally posted by @laeubi in #2991 (comment)