iPhone X support and iOS-related fixes

[endragor]

Starting from April 2018 Apple no longer accepts apps that do not
support iPhone X. For games this mainly means respecting the safe area,
unobstructed by notch and virtual home button. UI controls must be
placed within the safe area so that users can interact with them.

This commit:

• Adds OS::get_window_safe_area method that returns unobscured area of
  the window, where interactive controls should be rendered.

• Reorganizes how launch screens are exported - the previous way was
  incorrect and modern iPhones did not pick up the correct screens and
  because of that used a non-native resolution to render the game.

• Adds launch screen options for iPhone X.

• Makes launch screens optional in the export template. If not
  specified, a white screen will be used.

• Add App Store icon (1024x1024) export option as it now has to be
  bundled with the app instead of being provided in iTunes Connect.

• Fixes crash when launching games in iOS Simulator. It happened because
  controllerWasConnected callback came before the engine was
  initialized. Now in such case the controllers will be queued up and
  registered after initialization is done.

• Fixes issue with the virtual keyboard where for some reason
  autocorrection panel would intersect with the keyboard itself and not
  allow you to use the top row of the keyboard. This is fixed by
  disabling autocorrection altogether.

Closes #17358. Fixes #17428, #17331.

[akien-mga]

Build fails on Travis (Apple LLVM version 8.1.0 (clang-802.0.42)):

platform/iphone/gl_view.mm:84:10:{84:3-84:9}{84:12-84:38}: error: no viable overloaded '=' [2]
                insets = [_instance safeAreaInsets];
                 ~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk//System/Library/Frameworks/UIKit.framework/Headers/UIGeometry.h:15:16: note: candidate function (the implicit copy assignment operator) not viable: no known conversion from 'id' to 'const UIEdgeInsets' for 1st argument [2]
 typedef struct UIEdgeInsets {
                ^

[endragor]

Travis uses outdated Xcode by default. We need Xcode 9+ with iOS 11 SDK (Apple doesn't accept new submissions built with earlier versions). It seems osx_image needs to be set according to instructions here: https://docs.travis-ci.com/user/languages/objective-c/#Supported-Xcode-versions

[akien-mga]

@endragor Can you rebase on the master branch to get #18108?

[endragor]

@akien-mga done.

[hpvb]

cherry picked into 3.0.3