Before update callback

[mathieucarbou]

Introducing a callback to let the application aware when a refresh is requested, and if it is partial or not.

This helps improving performance by only refreshing some cards when the user reloads a page and not in a regular loop.

Example:

  dashboard.onBeforeUpdate([](bool changes_only) {
    if (!changes_only) {
      logger.debug(TAG, "Dashboard refresh requested");
      YaSolR::Website.initCards();
    }
  });

Then, we don't need to "prepare" any card before, but only when requested.

I.e. initCards() contain some card.update() calls for some application config, app info, etc. Basically values that are either never refreshed (app scope), or infrequently refreshed (i.e. a config could be updated through mqtt behind), so only refreshing them when the user refreshed the web page is acceptable.

[mathieucarbou]

Note: this PR contains #210 in a commit (I have #210 in my main branch to trigger CI)