Mark classes as 'open'

[charpov]

Nothing major and nothing urgent, but classes that are designed for extension, such as Tag or Suites should be marked as open to avoid warnings when compiling with the -source:future option.

[cheeseng]

@charpov Thanks for pointing out, that's definitely should be done, I have just submitted the following PR for this:

#2178

Do let us know if you notice more classes that should be marked as 'open'.

Thanks!