Show the build date in the screenshot gallery footer

The gallery gave no indication of how old the screenshots on the page were,
which is one of the remaining items on the GitHub Pages improvement list.

The generator now writes the time it ran alongside the screenshot data, and the
page renders it in the footer next to the existing links.

tools/test/generateAllScreenshots.py

@@ -167,6 +167,7 @@ def generateJavascriptFile():
 
     with open("screenshots/html/data.js", "w") as f:
         f.write("// Generated file, do not edit\n")
+        f.write("export const buildDate = %d;\n" % int(time.time()))
         f.write("export const screenshots = [\n")
         for line in data:
             f.write("[")