qdoc: Support offline search in .html files

Allow a project to embed a search bar in the generated documentation (.html) that works locally.

Use Case

This is useful for offline documentation, where the user opens the .HTML files on local disk.

In the Qt world, offline search is so far only available as part of Qt Help / Qt Assistant. Anyhow, this requires a) shipping and installing the tool, and b) limits the generated .HTML to the dialect Assistant supports. It would be arguably better to provide such a search also in a browser.

Also, Squish, TestCenter local documentation does feature such a search so far, so customers will also expect it to continue to be available.

Restrictions

Compared to an online search, the results could be limited to one help project. That is, search doesn't have to work across help project boundaries (like they are in the case of the Qt framework documentation).