Added NSApplicationDelegate's openFiles for wxOSX-Cocoa.

Added NSApplicationDelegate's openFiles for wxOSX-Cocoa.

openFiles (available since OS X 10.3) replaces using the openFile method. It allows for more convenient handling of multiple drops and knowing in advance how much files/folders are dropped instead of openFile with which you only get to respond to a single file/folder drop at a time. By default openFiles calls the newly added MacOpenFiles which calls MacOpenFile multiple times, so ordinarily the behaviour is backwards compatible (both on wxOSX Cocoa and Carbon).

The openFile instance method has been removed because it doesn't seem to be called anymore: neither when dropping a single file on the application in the dock or Finder nor when passed as a command-line argument.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68617 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775