Utils: Introduce a staging area for gui values

Accessing gui elements is very aspect-specific, whereas the
rules when and how values are synchronized are rather general.

Splitting the two concepts and having a permanent 'buffer' area
that is synchronized with the gui elements if and only if they
have been created helps to keep the boilerplate needed per-aspect
at a minimum.

This value could also serve as "model" in case we wanted to allow
multiple "views" in different places of the gui onto that aspect.

Change-Id: I34832512b99c53cb0e4df437ee9b4c5d17a2ad8f
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/debuggeractions.h

@@ -36,8 +36,8 @@ public:
     void writeSettings() const override;
 
 private:
-    void guiToInternal() override;
-    void internalToGui() override;
+    void guiToBuffer() override;
+    void bufferToGui() override;
 
     SourcePathMapAspectPrivate *d = nullptr;
 };