normalize png file name

this is a workaround for QTBUG-65543, but it also brings the file name
in line with the general pattern, so it's good on its own.

Change-Id: I1c17ece0538c2251d9d6c0df79ba38f358a69b72
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/libs/qmleditorwidgets/contextpanewidgetborderimage.ui

@@ -193,7 +193,7 @@
         </property>
         <property name="icon">
          <iconset resource="resources.qrc">
-          <normaloff>:/qmldesigner/images/tile-icon-hor - scale.png</normaloff>:/qmldesigner/images/tile-icon-hor - scale.png</iconset>
+          <normaloff>:/qmldesigner/images/tile-icon-hor-scale.png</normaloff>:/qmldesigner/images/tile-icon-hor-scale.png</iconset>
         </property>
         <property name="iconSize">
          <size>

src/libs/qmleditorwidgets/resources.qrc

@@ -35,7 +35,7 @@
         <file>images/aspect-crop-icon.png</file>
         <file>images/tile-icon-vert-crop.png</file>
         <file>images/tile-icon-hor-crop.png</file>
-        <file>images/tile-icon-hor - scale.png</file>
+        <file>images/tile-icon-hor-scale.png</file>
         <file>images/tile-icon-vert-scale.png</file>
         <file>images/anchor_bottom.png</file>
         <file>images/anchor_bottom@2x.png</file>