Add getStringLength() support for UITextField
Add const for getStringLength() function
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