Update AppDelegate.cpp

use getInstance replace :)
Fixed compilation errors when disabling CC_ENABLE_STACKABLE_ACTION
Merge pull request #6348 from qq573011406/patch-2

SimpleAudioEngine::sharedEngine() -> SimpleAudioEngine::getInstance() for cpp-template
Update the reference of submodule plugin-x.
Merge pull request #6341 from samuele3hu/develop_fix

fix two bugs found in the testing
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-x into develop
Merge pull request #6350 from natural-law/develop

Update the reference of submodule plugin-x.
Add tip message in setup.py for prompt the usage of some environment variable.
Merge pull request #6349 from dumganhar/config-fix

Fixed compilation errors when disabling CC_ENABLE_STACKABLE_ACTION
Merge pull request #6351 from natural-law/develop

Add tip message in setup.py for prompt the usage of some environment var...