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>