Add support for placeholder text

The Qt Quick editors, TextInput and TextEdit, should have built-in support for placeholder text. The following new properties should be added:

• placeholderText
• placeholder(Text?)Color

Qt Quick Controls 1 & 2 implement placeholder text support by creating an additional Text element, but it would be more efficient to re-use the same text node that is already used to show the edit text. Both never exist at the same time anyway. As a bonus, we have a chance to get the horizontal alignment right (QTBUG-55999).