Instead of duplicating the code from QmlEditorWigets we now subclass
and expose the gradient editing to qml.
The adaptor class is called GradientLineQmlAdaptor.
The library qmleditorwidgets contains widgets and image resources
what are shared between QuickToolBar and QmlDesigner
This library is created to make QuickToolBar independent from
QmlDesigner plugin.
All the widgets I moved to qmleditorwidgets are removed from
QmlDesigner in this patch