CodeAssist: Support only one quickfix assistant per editor class.

Currently we are just using the first that is found. Which could lead
to unexpected behavior. Adjust the API to make clear just one assistant
is supported per editor type.

Change-Id: I711e66b4c5c5f347118357a8bafa0ffc6d650f7e
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/cpptools/cppcompletionassistprovider.cpp

@@ -37,6 +37,10 @@ using namespace CppTools;
 // ---------------------------
 // CppCompletionAssistProvider
 // ---------------------------
+CppCompletionAssistProvider::CppCompletionAssistProvider(QObject *parent)
+    : TextEditor::CompletionAssistProvider(parent)
+{}
+
 bool CppCompletionAssistProvider::supportsEditor(Core::Id editorId) const
 {
     return editorId == CppEditor::Constants::CPPEDITOR_ID;