Merge 2.4 into master

Original pull requests:

* #2308 from jet47:gpu-tests-timeout
* #2309 from Nerei:fixed_cl_exe_hang_with_eigen
* #2314 from ilya-lavrenov:typo
* #2318 from gongzg:2.4
* #2320 from asmorkalov:ocv_readme_license_install
* #2321 from asmorkalov:android_run_all_tests
* #2323 from stuart8c:issue2601_2.4_fix_16bit_tiff_reading_on_big_endian_host
* #2329 from ilya-lavrenov:macosx_ogl
* #2331 from asmorkalov:android_native_activity_finish
* #2332 from jet47:gpu-fixes
* #2333 from asmorkalov:java_dead_code_fix
* #2335 from jet47:gpu-alphacomp-test
* #2342 from yashdv:android_doc_fix
* #2344 from kirill-kornyakov:replace-gittip-button
* #2348 from ilya-lavrenov:typo
* #2349 from jet47:fix-bug-3552