Cmake: Let the generator determine the toolchain

Otherwise we need to parse the cbp file, which happens only if the
buildconfiguration gets active. Also try to decouple a few internals a
little bit by using signals. The CMakeProject still handles a few things
directly instead of via signals, more to come eventually.

src/plugins/cmakeprojectmanager/cmakerunconfiguration.h

@@ -112,7 +112,7 @@ private slots:
     void setArguments(const QString &args);
     void baseEnvironmentChanged();
     void userEnvironmentChangesChanged();
-    void userChangesUpdated();
+    void userChangesChanged();
     void setWorkingDirectory();
     void resetWorkingDirectory();
 private slots: