Stop using NSApp in the OS X front end.

Stop using NSApp in the OS X front end.

As of OS X 10.10 (Yosemite), this causes a compile failure, which
according to the Internet is because NSApp is a generic 'id'
(Objective-C's answer to void *), which causes the compiler to select
the wrong method for [NSApp setDelegate:...] and give a spurious type
warning.

Fixed by using [NSApplication sharedApplication] throughout, or
rather, calling that once at startup and assigning the result into my
own global variable which will do as an NSApp replacement.

[originally from svn r10286]