Adding a few @2x icon variants

extension, splitbutton_vertical and unlocked. Also
some of the low-res pngs are replaced for consistency
and quality.

Change-Id: I2e7090efc75d4cb30f7759ab7b055b7026b47bd1
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>

src/plugins/coreplugin/core.qrc

@@ -5,6 +5,7 @@
         <file>images/clear@2x.png</file>
         <file>images/closebutton.png</file>
         <file>images/compile_error_taskbar.png</file>
+        <file>images/compile_error_taskbar@2x.png</file>
         <file>images/dir.png</file>
         <file>images/editcopy.png</file>
         <file>images/editcut.png</file>
@@ -48,7 +49,9 @@
         <file>images/undo.png</file>
         <file>images/unknownfile.png</file>
         <file>images/unlocked.png</file>
+        <file>images/unlocked@2x.png</file>
         <file>images/extension.png</file>
+        <file>images/extension@2x.png</file>
         <file>images/darkclosebutton.png</file>
         <file>images/darkclosebutton@2x.png</file>
         <file>images/category_core.png</file>
@@ -84,6 +87,7 @@
         <file>images/splitbutton_closetop.png</file>
         <file>images/splitbutton_closetop@2x.png</file>
         <file>images/splitbutton_vertical.png</file>
+        <file>images/splitbutton_vertical@2x.png</file>
         <file>images/panel_manage_button.png</file>
         <file>images/sidebysidediff.png</file>
         <file>images/textdiff.png</file>

src/plugins/projectexplorer/projectexplorer.qrc

@@ -44,6 +44,7 @@
         <file>images/window.png</file>
         <file>images/stop_small.png</file>
         <file>images/disabledbuildstep.png</file>
+        <file>images/disabledbuildstep@2x.png</file>
         <file>images/unconfigured.png</file>
         <file>images/targetleftbutton.png</file>
         <file>images/targetrightbutton.png</file>