Merge as much as possible of the different Q*Control classes

When Qt Widget was split into a separate module, we made four different classes doing much of the same thing: QWidgetTextControl, QWidgetLineControl, QQuickTextControl, and QQuickLineControl.

While they have much of the same functionality, they don't share any code, meaning that many bugs have to be fixed in four different locations.

A more robust structure would be to make Qt Gui have a QInputControl which is inherited by QTextInputControl and QLineInputControl which in turn are inherited by the Qt Quick and Qt Widgets-specific implementations.

It's a bit of work but will save us some down the line.