The Terminal could only split panes inside one tab. Split Right / Split Down now create tab groups, each with its own tabs, + and ▾; tabs can be moved between groups by dragging or Split and Move Right/Down, Unsplit merges a group back, and closing a group's last tab focuses the neighbouring group. Split actions are registered as actions and appear in the tab context menu.
Fixes#774
Co-authored-by: StarryKira <[email protected]>