First stab at text/line edit rendering. Reviewed by Zack.

* Introduce new "Widget::setParentWidget" logic similar to "setWindowHandle" (windows backend) and/or "setDrawable" (gdk backend), which will create the actual widgets of ie. a LineEdit and associate the right parent with it.

Once a ie. <input type=password> object is constructed a new PlatformLineEdit will be created by the RenderText* objects, and ScrollView::addChild(Widget*) will be called. There we now know the parent widget and can properly construct our QLineEdit, as PlatformLineEdit::setParentWidget() actually handles the creation.

I hope this is flexible enough. Comments please :-)

* Remove parentQObject stuff from FrameQt (obsolete now)
* Adapt PlatformLineEdit/PlatformTextEdit and their *Qt implementations to that logic.