Provide terminal tabs.

Depends on #956

This requires basic support for the tree structure windowing.

• per output window, only one session is being displayed (no session management in this ticket yet).
• one session can contain multiple tabs (by default being displayed in the bottom line)
• one tab can contain multiple panes (tree structured)
• one pane contains exactly one single terminal view

Contour: Actions

• NewTab (Ctrl+Shift+T)
• SelectNextTab (Ctrl+Shift+PgDn)
• SelectPrevTab (Ctrl+Shift+PgUp)
• SelectTab(index) (Ctrl+Alt+)
• CloseTab (Ctrl+Shift, C)

TODO Items

• TerminalTree: terminal tree data structure (session/panel/pane)
• action: rename session (default: indexed)
• replace single-TerminalView with TerminalTree
• action: rename panel (default: indexed)
• action: rename pane (default: terminal title)
• action bindings