Fix warning when compiling against OS X 10.10 SDK.

Something broke NSApp compatibility object in 10.10 and its setDelegate:
expects NSFileManagerDelegate, not NSApplicationDelegate, and the
compiler warns about. Use the equivalent [NSApplication
sharedApplication] object to fix this.

Also cast appcontroller to the expected type, because passing untyped
object to setDelegate: yields a warning as well.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77940 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775