Using NSApp directly seems to confuse something, as the compiler
was expecting an id<NSFileManagerDelegate>. Switched to using
[NSApplication sharedApplication], and specified the delegate
protocol on the window class as well.
Similar to https://bugzilla.gnome.org/show_bug.cgi?id=738740
Until gcc and GNUStep properly support Objective-C blocks and other
"new" features of Objective-C we can't properly support them without
making the code much more ugly.
https://bugzilla.gnome.org/show_bug.cgi?id=739152