Tab system redesign

[ERSUCC]

Simplified and improved the code managing the interface, info, and code tabs, as well as the separate code tab window. The following bugs should now be fixed:

• Find dialog refocuses main window on closing even when opened from the separate code tab window
• Switching between the main window and the separate code tab window without switching the selected tab in either window doesn't correctly change the focus
• Clicking on a tab doesn't always focus it
• Opening the separate code tab window for the first time or after opening a saved included file refocuses the main window

Also adds a menu bar for the separate code tab window, and adds a preference for opening the separate code tab window on startup.

Note: LevelSpace extension pull request #147 must be integrated along with this pull request to ensure compatibility. The GitHub tests will not pass until it is integrated but it has been confirmed that they will pass if both are integrated.