merge r31039 from trunk to branch 2.7

bug 3212 fixed: add CSS rule to support themes like Modus


git-svn-id: http://piwigo.org/svn/branches/2.7@31040 68402e56-0260-453c-a942-63ccdbb3a9ee

plugins/language_switch/style.css

@@ -20,4 +20,10 @@
 }
 #languageSwitchBox a:hover {
   color:#ddd;
-}
+}
+
+[class*="langflag-"] {
+  overflow: hidden;
+  text-indent: -9999px;
+  vertical-align: bottom;
+}