Make thumbnail min/max size configurable via theme

src/themes/Default.qpl

@@ -352,11 +352,16 @@ chat:
         string styleInclude:
             '<style type"text/css">\n' + styleSheet + '\n</style>\n'
 
-        // TODO rename
+        // Prefered minimum width of file messages
         int fileMinWidth: 256
-        int thumbnailWidth: 256
-        int videoWidth: 640
-        int audioWidth: 320
+
+        // Don't scale down thumbnails below this size in pixels, if
+        // the becomes too small to show it at normal size.
+        size thumbnailMinSize: Qt.size(192, 192)
+
+        // How much of the chat height thumbnails can take at most,
+        // by default 0.3 for 30%.
+        real thumbnailMaxHeightRatio: 0.3
 
     daybreak:
         color background: colors.strongBackground