button style: add pressed state

tt-rss.css

@@ -2251,6 +2251,10 @@ div#auxDlg a {
 	color : #4684ff;
 }
 
+button:active {
+	background-image : url("images/button_pressed.png");
+}
+
 button, select {
 	background-color : transparent;
 	background-repeat : repeat-x;