update label and placholder on programatic update of text (after init)

reusing `-[*TextDidChange:]` handlers to update the label and placeholder when manually changing the text via `-[setText:]`

Without this, something like `self.textView.text = nil;` would not properly update the placeholder and floating label

---

Also implemented the default initializer for `UITextView` which allows initializing with a custom `NSLayoutManager` and, therefore, it's associated TextKit compatriots (ie. `NSTextStorage` and `NSTextContainer`)