- 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).
