Update templates and example Xcode project files to indicate they've
been tested for Xcode 8.3 compatibility.

Gallery and stocks:
* Add 20x20 notification icons.
* Remove unused 76x76 (3x scale) icon.

Other examples: update JSON to Xcode 8.2.1 defaults

Fixes warnings for missing/unused icon assets in Xcode 8.2.x

Required to support Flutter SDK paths with spaces.

Subclassing FlutterAppDelegate is not required for real-world apps, but
it's what 'flutter create' generates, and applies the default behaviours
that most real-world apps will want.

This avoids Xcode complaining at us about unused xcconfig files (for the
time being).

As of Xcode 7, Apple recommends setting the bundle identifier in the
PRODUCT_BUNDLE_IDENTIFIER build variable and setting CFBundleIdentifier
to that variable in Info.plist.

As of Xcode 8, CLANG_WARN_INFINITE_RECURSION and CLANG_WARN_SUSPICIOUS_MOVE
are recommended settings.

* Update examples to lower the deployment target on iOS.

* Remove stray PRODUCT_BUNDLE_IDENTIFIER

This was erroneously added and overrides
the bundle for the gallery causing signing
to fail in my setup.

@chinmaygarde

* Remove PRODUCT_BUNDLE_IDENTIFIER from all Runner pbxproj's

This patch changes the default `flutter create` command to generate a
HelloServices-style app on iOS. As a consequence, the Atom integration now
fully works with HelloServices.

This file is now app.dylib.

Cleans up a few style nits in hello_services and adds support for automatic
JSON encoding and decoding to the HostMessages interface.

The hello_services example can now be built both for Android and iOS.

Currently this service just returns mock data, but it exercises the plumbing.

This example shows how to build a basic app using a custom Xcode project.
Future patches will bring the functionality up to par with hello_android.