Enable to set/change flags in SME::Element

This can be used for example in smt to display
external sourced sub-elements (like a nested
statemachine) and disable them for editing.

Change-Id: Ie27b40b97f250f72d4f6bf68436f8b928a70457f
Reviewed-on: https://codereview.kdab.com/13852
Reviewed-by: Continuous Integration <build@kdab.com>
Reviewed-by: Volker Krause <volker@kdab.com>
Fix use editable-state in propertyeditor

Change-Id: Icc64e468ffa2085279356304bf9a0d1a3c69a175
Reviewed-on: https://codereview.kdab.com/13853
Reviewed-by: Continuous Integration <build@kdab.com>
Reviewed-by: Volker Krause <volker@kdab.com>
Enable control on attribute-level what to export

* Introduce a writeAttribute method and use that one
  rather then checking everywhere for value.isEmpty
  and then pipe indention, key, value and newline
  into the stream. More short, logical and consistent
  to use an own method for that.

* Introduce the com.kdab.KDSME.DSMExporter.implBindingNames
  property which allows to define a list of attribute-names
  which are actually implementation. This is used to know
  where the state's attribute was defined, in the
  implementation or its base (as in external file).

  This makes it possible to set the attribute source and on
  export restore the proper origins.

Change-Id: I44e742ae2d644b1b23914e80164adafa21fdd852
Reviewed-on: https://codereview.kdab.com/13854
Reviewed-by: Continuous Integration <build@kdab.com>
Reviewed-by: Volker Krause <volker@kdab.com>