Add option for globally changing the base environment for tools

Fixes: QTCREATORBUG-22123
Change-Id: I00a5bbdf92e5dab513cb12f3518a2abb1adbb9cd
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/coreplugin/icore.h

@@ -148,6 +148,7 @@ signals:
     void coreAboutToClose();
     void contextAboutToChange(const QList<Core::IContext *> &context);
     void contextChanged(const Core::Context &context);
+    void systemEnvironmentChanged();
 
 public:
     /* internal use */