Consider preview thumbnails to calculate msg width

Also move thumbnails width setting from settings.json to the theme

src/themes/Default.qpl

@@ -307,6 +307,8 @@ chat:
         string styleInclude:
             '<style type"text/css">\n' + styleSheet + '\n</style>\n'
 
+        int thumbnailWidth: 256
+
     daybreak:
         color background: colors.strongBackground
         color text:       colors.text