Add getStringLength() support for UITextField
Add const for getStringLength() function
Merge pull request #7004 from Dhilan007/v3-labelfix

close issue#5158:Label's display may go bonkers if using outline feature.
[ci skip]
Merge branch 'improvement/COC-3' of https://github.com/zhouxiaoxiaoxujian/cocos2d-x into improveUITextField

Conflicts:
	cocos/ui/UITextField.h
Merge pull request #7015 from andyque/improveUITextField

add getStringLength to UITextField