[ticket/12406] Fix variable description of core.viewtopic_modify_page_title

Variable page_title should be described as "Title of the viewtopic page".

PHPBB3-12406
[ticket/12097] Validate_data() should work with class method

https://tracker.phpbb.com/browse/PHPBB3-12097

PHPBB3-12097
[ticket/12097] Bug fix and adding unit test

PHPBB3-12097
Merge pull request #2325 from n-aleha/ticket/12406

[ticket/12406] Fix variable description of core.viewtopic_modify_page_ti...

* n-aleha/ticket/12406:
  [ticket/12406] Fix variable description of core.viewtopic_modify_page_title
Merge pull request #2359 from Nicofuma/ticket/12097

[ticket/12097] Validate_data() should work with class method

* Nicofuma/ticket/12097:
  [ticket/12097] Bug fix and adding unit test
  [ticket/12097] Validate_data() should work with class method