Add 2x icons for retina displays.

This change updates the mode selector/fancyactionbar
in Qt Creator with new high-resolution icons.

Set Qt::AA_UseHighDpiImages on Qt 5.1 and higher to
make QIcon generate high-dpi pixmaps. Add "@2x" mode
images.

Update StyleHelper::drawIconWithShadow to handle high-dpi
pixmaps. The shadow-drawing algorithm is kept in device
pixels.

Change-Id: I411b7a24e534a2d75a1bbdc4d10219dcbea26bed
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/plugins/coreplugin/coreconstants.h

@@ -204,6 +204,7 @@ const char ICON_CLOSE_SPLIT_RIGHT[]  = ":/core/images/splitbutton_closeright.png
 const char ICON_FILTER[]             = ":/core/images/filtericon.png";
 const char ICON_LINK[]               = ":/core/images/linkicon.png";
 const char ICON_QTLOGO_32[]          = ":/core/images/logo/32/QtProject-qtcreator.png";
+const char ICON_QTLOGO_64[]          = ":/core/images/logo/64/QtProject-qtcreator.png";
 const char ICON_QTLOGO_128[]         = ":/core/images/logo/128/QtProject-qtcreator.png";
 
 const char WIZARD_CATEGORY_QT[] = "R.Qt";

src/plugins/coreplugin/fancyactionbar.qrc

@@ -1,10 +1,15 @@
 <RCC>
     <qresource prefix="/fancyactionbar">
         <file>images/mode_Debug.png</file>
+        <file>images/mode_Debug@2x.png</file>
         <file>images/mode_Edit.png</file>
+        <file>images/mode_Edit@2x.png</file>
         <file>images/mode_Output.png</file>
         <file>images/mode_Project.png</file>
+        <file>images/mode_Project@2x.png</file>
         <file>images/mode_Reference.png</file>
+        <file>images/mode_Reference@2x.png</file>
         <file>images/mode_Design.png</file>
+        <file>images/mode_Design@2x.png</file>
     </qresource>
 </RCC>