Core: Toolbar icons improvements

Change the type of OutputPaneManager::m_maximizeIcon and
::m_minimizeIcon from QPixmap to QIcon to have the automatic @2x
loading.

Small retouches of the graphics: arrowup and arrowdown got the same
color and shadow like the other toolbar icons. The line width of
button_close got adjusted.

Change-Id: Ia985ae23b8c9f301fecee64e8283c6e556e6e0c2
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/outputpanemanager.h

@@ -132,8 +132,8 @@ private:
     QStackedWidget *m_outputWidgetPane;
     QStackedWidget *m_opToolBarWidgets;
     QWidget *m_buttonsWidget;
-    QPixmap m_minimizeIcon;
-    QPixmap m_maximizeIcon;
+    QIcon m_minimizeIcon;
+    QIcon m_maximizeIcon;
     bool m_maximised;
     int m_outputPaneHeight;
 };