Add getter and setter for TextColor to Text Widget

Add a function to set text color property for label inside Text widgets.
The "Color Tint" option in CocoStudio sets the color for the entire widget (setColor method); setting text color this way will cause outline colors for Text widget to be incorrect.
Does setTextColor method change the label's contentSize? Why should we call `updateContentSizeWithTextureSize ` after changing Label's textColor