Added stub files, cocoa/AppDelegate.h and cocoa/AppDelegate.m, and an XML Interface...

Added stub files, cocoa/AppDelegate.h and cocoa/AppDelegate.m, and an XML Interface BUilder file, cocoa/Base.lproj/MainMenu.xib, so that it's easier to regenerate MainMenu.nib if making substantial changes to the Cocoa menus.  Comments in cocoa/AppDelegate.m outline the procedure.  The former src/cocoa/en.lproj/MainMenu.nib (as a directory) has been replaced by src/cocoa/Base.lproj/MainMenu.nib.  The version in this commit was generated with Xcode 11.3 with a deployment target of 10.8.  There is no longer a separate nib, src/cocoa/ja.lproj/MainMenu.nib, for Japanese.  Instead have src/cocoa/ja.lproj/MainMenu.strings to specify the Japanese versions of the menu labels.  Since Base localization is supposed to only be available in 10.8 and later, set the deployment target flags in Makefile.am to be 10.8 rather than 10.7.