QmlDesigner.PropertyEditor: Fixing expressions and tr checkbox

The handling of expressions in states was broken in conjunction with states.

This patch fixes a couple of related issues:

*  The notifier of isTranslated (PropertyEditorValue) has to be
   expressionChanged, since this property depends on the expression.

* isTranslated() has to return also true if there is only an expression
  in the base state. If the string is translated in the base state it
  should be translated in other states, too.

* In states the expression set in createPropertyEditorValue() has to be the
  expression of the current stateif there is one.
  We can not just fall back to instanceValue.

* I did a couple of fixes in LineEdit.qml. We cannot bind checked directly,
  since the binding will be deleted when checked is changed by the control.
  I removed all code regarding transactions. We react to onEditingFinished,
  so it is not required. The translated state is properly preserved now, when
  setting the value.

Task-number: QTCREATORBUG-10674
Task-number: QTCREATORBUG-10791
Change-Id: I95721711a37e63c4c7c38e275089d6de0bb92bec
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.ItemLibrary: Add default title for GroupBox

Change-Id: Id1ff15180466631ebf8f984c998417613ba97de3
Reviewed-by: Marco Bubke <marco.bubke@digia.com>