Ensure terminal pane shell location correct when opened

[jeremypw]

Fixes #1523

• [x] Save terminal-pane state explicitly - destroy signal unsuitable
• [x] Sync terminal-pane state when hidden

Consistency of behaviour was improved by using the same MainWindow action to set the active project whether from the project chooser or the context menu and dropping a signal/handler that did much the same thing. Responsibility for updating the git_manager and collapsing the other projects transferred to MainWindow.

Note: To test you must manually set the active project and close/reopen the app in order to ensure the settings are updated.