Usage of imread(): magic number 0, unchecked result

* docs: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()

* samples, apps: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()

* tests: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()

* doc/py_tutorials: check imread() result

Expose CGImage <-> Mat conversion for iOS platforms

* Add apple_conversions to framework builds

This exposes CGImage <-> Mat conversion.

* Export Mat <-> CGImage methods on iOS targets

* Add CGImage converters to iOS objc helper class

* Add CF_RETURNS_RETAINED annotations to methods returning CGImageRef

- 'AdditionalImports' can be removed from misc/objc/gen_dict.json
- avoid using of legacy 'opencv.hpp'

Mat conversions for macOS/AppKit

* Extract CoreGraphics conversion logics from ios_conversions.mm to apple_conversions.h, apple_conversions. Add macosx_conversions.mm

* Add macosx.h

* Add Mat+Conversions.h and Mat+Conversions.mm

* Delete duplicated declaration from apple_conversion.mm

* Use short license header

* Add compile guard

* Delete unused imports

* Move precomp.hpp import from header to implementation

* Add macosx.h to skip headers

* Fix compile guard condition

* Use short license header

* Remove commented out unused code

Objc binding

* Initial work on Objective-C wrapper

* Objective-C generator script; update manually generated wrappers

* Add Mat tests

* Core Tests

* Imgproc wrapper generation and tests

* Fixes for Imgcodecs wrapper

* Miscellaneous fixes. Swift build support

* Objective-C wrapper build/install

* Add Swift wrappers for videoio/objdetect/feature2d

* Framework build;iOS support

* Fix toArray functions;Use enum types whenever possible

* Use enum types where possible;prepare test build

* Update test

* Add test runner scripts for iOS and macOS

* Add test scripts and samples

* Build fixes

* Fix build (cmake 3.17.x compatibility)

* Fix warnings

* Fix enum name conflicting handling

* Add support for document generation with Jazzy

* Swift/Native fast accessor functions

* Add Objective-C wrapper for calib3d, dnn, ml, photo and video modules

* Remove IntOut/FloatOut/DoubleOut classes

* Fix iOS default test platform value

* Fix samples

* Revert default framework name to opencv2

* Add converter util functions

* Fix failing test

* Fix whitespace

* Add handling for deprecated methods;fix warnings;define __OPENCV_BUILD

* Suppress cmake warnings

* Reduce severity of "jazzy not found" log message

* Fix incorrect #include of compatibility header in ios.h

* Use explicit returns in subscript/get implementation

* Reduce minimum required cmake version to 3.15 for Objective-C/Swift binding