make terminal emulator configurable centrally

this includes changing the runInTerminal.command command line.
the terminal setting mock from the debugger plugin is gone again.

src/plugins/coreplugin/generalsettings.h

@@ -59,6 +59,9 @@ private slots:
     void resetInterfaceColor();
     void resetExternalEditor();
     void showHelpForExternalEditor();
+#ifdef Q_OS_UNIX
+    void resetTerminal();
+#endif
 
 private:
     Ui_GeneralSettings *m_page;