Add session save and restore menu actions

[vacu9708]

Summary

Add terminal context menu actions for saving and restoring a lightweight session layout.
This introduces a new Session submenu in the terminal popup menu with:

• Save session
• Restore session

Saving persists the current Terminator layout using the existing layout serialization path, including each terminal’s current working directory. Restoring loads the saved session layout in the current process and replaces the window where restore was requested.

Scope

This restores :

• windows/panes/tabs layout
• working directories

Implementation Notes

• Saves the session as a reserved layout named __last_session__.
• Hides __last_session__ from the normal Layouts submenu.
• Restoring creates the saved layout, then destroys the initiating window so the session replaces the current window rather than opening an extra one.

Testing

• python -m py_compile terminatorlib/terminator.py terminatorlib/terminal_popup_menu.py
• pytest -q terminatorlib/borg.py terminatorlib/config.py terminatorlib/cwd.py terminatorlib/factory.py terminatorlib/util.py tests/test_borg.py tests/test_signalman.py

[mattrose]

There's already a plugin that does this.