Make method naming more consistent.

* Use id() for methods returning a string used to represent
    some type of object.
  * Use displayName() for strings that are meant to be user
    visible.
  * Quieten some warnings while touching the files anyway.
  * Move Factories to their products in the plugins where that
    was not done before.

Reviewed-by: dt

src/plugins/designer/formwindoweditor.cpp

@@ -263,9 +263,9 @@ Core::IFile *FormWindowEditor::file()
     return m_file;
 }
 
-const char *FormWindowEditor::kind() const
+QString FormWindowEditor::id() const
 {
-    return C_FORMEDITOR;
+    return QLatin1String(C_FORMEDITOR_ID);
 }
 
 QString FormWindowEditor::displayName() const