Move main window to its own nib
Remove unused protocols from app delegate
Add MainWindow.xib to project
Use a custom field editor that hides its find action support

The standard field editor supports find actions but disables their menu
items in its validateUserInterfaceItem: implementation. This conflicts
with the custom find implementation in the window controller. By hiding
NSTextView's find action support in the custom field editor this
conflict is resolved without needing to chain the application-wide menu
items and breaking find action support in any future text views that
might be added to other windows.
Switch find menu items to performTextFinderAction:, add hide find bar
Explain use of windowWillReturnFieldEditor:toObject:
Use menu items for find actions
Declare helper functions static
Add STAMainWindowFieldEditor to project
Support cancelOperation: for the find bar
Hide the find bar for new selections rather than new searches
Fix parameter name for searchAgain:
Add find forward/back buttons, match standard find bar layout
Support NSTextFinderActionSetSearchString
Find pasteboard support for find bar