Find in Files extension searching should search project extensions

[DavidKinder]

The Windows front-end has now implemented https://github.com/ganelson/inform-evolution/blob/main/proposals/0017-apps-and-extensions.md, which makes per-project extensions the primary way to use extensions. However, at the moment the Find in Files dialog just searches the (now legacy) shared extensions area under "My Documents\Inform\Extensions".

The Find in Files dialog will be changed to instead search extension sources in the project's extensions, under the project's materials directory.

In addition, there will be a new option to search the generated extension documentation for the project. This will be a new checkbox "Extensions" under "Where to look / Documentation".

[DavidKinder]

As of commit af8c3ca, the first part of this has been done, in that when searching extensions, all extensions are searched: built-in ones, legacy folder ones, and those in the project's materials folder.