This is needed to build when some dependencies are installed into
locations outside of /usr.

This brings us in line with the [build
API](https://github.com/cgwalters/build-api) used by flatpak-builder.

This reworks the file button in the help browser so that it opens the file
browser on whatever directory we visited last (as given by the current_dir
variable), or the doc directory by default.

This reverts commit ac536d2f.

Unfortunately we must rely on these in Purr Data. It's impossible to
enumerate every single function signature, especially given some are
dynamically loaded at runtime.