Add support for HTML5 placeholder in views exposed filters

Drupal core views finally has support for the "html5 placeholder" in views (see the related issue).

It has default support for the core view displays. But Search API doesn't support this yet for "fulltextsearch" and will need to add support for it.

At least that's what I understand from these 2 comments:
Question:

I looked in to the proposed approach in #5, but I don't think this is a good solution.
Lets assume we add the placeholder attribute in the plugins "StringFilter" and "NumericFilter". This fixes the problem for Drupal core filters. But for example if we have a "SearchApiFulltext" provided by SearchApi then we won't be able to configure the placeholder because this plugin does not provide it.

There are a lot of other modules that expose custom filter plugins and the placeholder attribute should be configurable on all of them without the developers of the module having to alter their plugin.

Answer:

I would lean toward the approach in #5. As we've already established, placeholder text only applies to particular form elements. As far as I can see, there's no way for us to generically apply it at the FilterPluginBase level. It depends on the widget provided by each individual filter plugin, and only that plugin is going to know for sure where and how a placeholder should be applied.

To take an example out of core, if you're using a NumericFilter with the BETWEEN operator, the value form contains both a "Min" and a "Max" field, not just a singular "Value" field. You may well want a placeholder for both of those fields, but FilterPluginBase is going to have no idea how to accommodate that — only NumericFilter does.

And yes, that means that any contributed module's filter derived from FilterPluginBase directly (like SearchApiFullText) will not magically inherit placeholders. Not ideal, but I'm not sure its avoidable.