Split general settings page in two

It was too crowded to add anything to it. Split it into "interface" and
"system" related pages.

Change-Id: I2aa7984de4d672ca000e991b0cfb32a3f706c988
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>

src/plugins/coreplugin/mainwindow.h

@@ -76,6 +76,7 @@ class StatusBarManager;
 class VersionDialog;
 class WindowSupport;
 class SystemEditor;
+class SystemSettings;
 
 class MainWindow : public Utils::AppMainWindow
 {
@@ -174,6 +175,7 @@ private:
     QMap<QWidget *, IContext *> m_contextWidgets;
 
     GeneralSettings *m_generalSettings;
+    SystemSettings *m_systemSettings;
     ShortcutSettings *m_shortcutSettings;
     ToolSettings *m_toolSettings;
     MimeTypeSettings *m_mimeTypeSettings;