- EditorExportPlugin's _export_begin accepts all the arguments related
   to the current export (is_debug, path, flags).

 - EditorExportPlugin API is extended with methods allowing to configure
   iOS export: add_ios_framework, add_ios_plist_content,
   add_ios_linker_flags, add_ios_bundle_file.

 - iOS export template now contains Godot as a static library so that
   it can be linked with third-party Frameworks and GDNative static
   libraries.

 - Adds method to DirAccess for recursive copying of a directory.

 - Fixes iOS export to work with Xcode 9 (released recently).