Core: High-Dpi rendering of ManhattanStyle PE_PanelLineEdit

New @2x variants for inputfield.png and inputfield_disabled.png. Loading
them with Utils::StyleHelper::dpiSpecificPixmap(). Small adjustments for
resolution independent drawing of the focus/hover rect.

Change-Id: I6e1322e597c1fca7fc4ff7cce8801e99ce03e57a
Reviewed-by: Eike Ziller <eike.ziller@digia.com>