Add rounded corner support to TextBox and PasswordBox

Although the widgets used inside TextBox and PasswordBox support rounded corners, these widgets themselves do not provide it.

By adding the radius property to the TextBox and PasswordBox, these widgets become capable of supporting rounded corners, which are a simple nice ability to give to themes.
I am very new to the QML world, and I do not know yet which are the best practices. If an alias is better suited I can make the change.