[refactor] Consolidate `UITextView` and `UITextField` APIs.
[refactor] rename classes that start with `UI` to `CCUI`, since that prefix is reserved by Apple.
[bugfix] setting the placeholder for `CCUISingleLineTextField` was missing in `createSingleLineTextField`
[bugfix] In `MultiLineTextView`, if `placeholder` changed, there doesn't seem to be a deterministic way of updating the `placeholderLabel`. We are relying on `drawRect:`. Instead, `placeholder` is now a computed property on top of `placeholderLabel.text`.
[bugfix] If the user wants multi-line secure input, we should set it, since it is possible, right?
[minor] `[viewWithTag:999]` was used instead of calling the property itself.
[minor] `drawRect:` was used to load properties, when it is normally in the getter.
[minor] fix ObjC code style
zilongshanren