V2.4.0: fixes, Japanese parsing

This version 2.4.0 brings fixes to a bunch of annoying problems. Most of all, you should no longer see popups any time you leave a page, and Japanese parsing is now officially supported on Windows.

Added

• The external dependency for Japanese parsing, MeCab, is now more easily detected on Windows and Linux.
• Better integration of Docker.

Changed

• You must not only include pgm.js to ask the user before exiting but also the new function ask_before_exiting.
• splitCheckText was split into smaller functions.
• Refactored edit_languages.php with functional paradigm.

Fixed

• Many pages were asking before exiting while it was unnecessary.
• Made bigger buttons and uniformed the presentation of edit_texts.php,
  edit_archivedtexts.php, long_text_import.php and do_feeds.php. It solves
  issue #29.
• Japanese texts were ill parsed on Windows. The issue #23 is now solved!
  @rc-ops、ありがとう〜!
• The number of texts, expressions and so on was always 0 in edit_languages.php.
• On Windows, annoying notices were often displayed in edit_languages.php.

Removed

• The HTML code documentation is no longer included in the dev branch.
• The unnecessary JS files were removed since they were all merged in js/.

Full Changelog

• 2.3.0...2.4.0