QmlDesigner.propertyEditor: more css files for buttons

share/qtcreator/qmldesigner/propertyeditor/Qt/styledbuttonleft.css

+QPushButton {
+    border-image: url(:/qmldesigner/images/button-normal-left.png);
+ }
+ 
+  QPushButton:pressed {
+    border-image: url(:/qmldesigner/images/button-pressed-left.png);
+ }
+ 
+ QPushButton:checked {
+    border-image: url(:/qmldesigner/images/button-pressed-left.png);
+ }

share/qtcreator/qmldesigner/propertyeditor/Qt/styledbuttonmiddle.css

+QPushButton {
+    border-image: url(:/qmldesigner/images/button-normal-middle.png);
+    padding: 6px;
+ }
+ 
+  QPushButton:pressed {
+    border-image: url(:/qmldesigner/images/button-pressed-middle.png);
+ }
+ 
+ QPushButton:checked {
+    border-image: url(:/qmldesigner/images/button-pressed-middle.png);
+ }

share/qtcreator/qmldesigner/propertyeditor/Qt/styledbuttonright.css

+QPushButton {
+    border-image: url(:/qmldesigner/images/button-normal-right.png);
+    padding: 6px;
+ }
+ 
+  QPushButton:pressed {
+    border-image: url(:/qmldesigner/images/button-pressed-right.png);
+ }
+ 
+ QPushButton:checked {
+    border-image: url(:/qmldesigner/images/button-pressed-right.png);
+ }