v2.10.0: fully async saving, safe upgrading, reusable icon(s), UI updates (including tabs), fixes, and more

Changes summary:

• Behavior improvements:
  • Upgrading wizard is now more instructive and prevents losing content when trying to upgrade with some savers;
  • <<version>> now also shows the number of a "nightly" build.
• UI updates (see visual summary below the changelog):
  • Tabs now use common background (instead of gray), don't "sink" into the content and are not broken into 2 lines;
  • Remove an excessive link in GettingStarted;
  • Search button now looks the same as other buttons in the sidebar;
  • Nice toggle backstage button's padding;
  • Wizard status now supports html (links) and looks nicely when multiline;
  • Added bottom padding to tiddlers.
• Fixes:
  • Make Array.prototype.find fall back to the native implementation if a predicate is provided;
  • Story.prototype.setTiddlerField failed for tags (affected IntelliTaggerPlugin);
  • Checkboxes in the Disabled column of <<plugins>> should show correct state;
  • Don't overwrite Array.prototype.indexOf if it's defined.
• Hackability:
  • Introduces fully async saving (disabled by default: unset chkPreventAsyncSaving to enable):
    • new general method tw.io.saveFile (use it over the global saveFile), similar to tw.io.loadFile;
    • saveMain now supports callback (tries async saving if it is passed and chkPreventAsyncSaving is unset);
    • new method tw.io.saveMainAndReport separates the whole "load original, update, and save" thing, so may be a better target of decorating/overriding than saveChanges in some plugins/savers.
  • Icons (the close icon) are now stored in tw.assets.icons and are reusable.
• Deprecated: Array's .setItem, Java IO (only javaSaveFile and javaLoadFile are left, returning null).
• Infrastructure:
  • updates of some autotests, removed legacy reporter;
  • fix: freeze TW5 version used for building (versions 5.2.2 and above break things) in package.json.
• Lingo updates (for translations)
• Docs:
  • repairing links in [[Examples]] and [[TutorialsAndGuides]] and adding the Volvo page and Dutch usermanual there;
  • more compact and accurate [[Setting up saving]] and related tiddlers ([[Advanced download options]], details for some browsers);
  • up-to-date instructions for updating (both inside TW and in [[HowToUpgrade]]);
  • rewrite of [[slider macro]], mentions of plugins introducing additional filters;
  • updates of [[HelloThere]] and [[TiddlyWiki5]];
  • various minor updates, including site plugins.
• Dev docs:
  • several new articles about using git and GitHub and contributing;
  • new 'Releasing and updating the site' section in the build readme;
  • clean up of DeveloperDocumentation, updating links, and also various updates of the dev space for its further editing.

Acknowledgements for contributions: to Okido, Mark.

See the full changelog here: #294