Add a property for the text colour of AbstractButton

Changing the colour of text in an abstract button is common but in Qt Quick Controls 2 it's extremely difficult. One needs to restyle the button instead of setting a simple value. This either forces using a private API in the form of IconLabel or essentially rewriting the entire component, especially if it has an icon as well. Button, CheckBox, DelayButton, ItemDelegate, MenuBarItem, MenuItem, RadioButton, Switch, and TabButton are affected as they all derive from AbstractButton.