Better display of images, especially generated trees

Images now have a default maximum width of 100%.

Generated tree images now must be marked with class `gentree' and will
be color-inverted (i.e. their black parts will be white, while the
transparent background remains transparent) with dark themes.

The TOC and the language dropdown have been inverted.

web/dart/reorganize_html.dart

@@ -126,12 +126,12 @@ Future<Element> makeNavbar() async {
           ..attributes['href'] = '/'
           ..append(makeIcon(['fas', 'fa-home']))));
 
-  // TOC icon
-  navbar_content.append(await makeTocDropDown());
-
   // Add languages
   navbar_content.append(await makeLanguages());
 
+  // TOC icon
+  navbar_content.append(await makeTocDropDown());
+
   // Share icon
   navbar_content.append(await makeShare());