fixed segfaults caused by preset_hub_read and preset_hub_readpreset (may want to revisit later to disable the crflag from the binbuf_read_via_canvas and consider implications of potentially running into an A_SEMI which currently does not appear to be likely, or dare I say, possible?)

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.