Using XDG's user dir settings to determine where pictures are
stored for a user (e.g., "~/Pictures" -- used as a fallback).
May be overridden using "--exportdir".
Also, while I was updating some docs, replace references to
"Mac OS X" with "macOS", the new name of that OS these days.
They're in a new location over on John P.'s website
(should we purhapse integrate them into Tux Paint proper?)
(h/t sechidis & whoever on #tux4kids IRC channel who helped them)
Documenting Mark Kim's recent changes:
* Less CPU-hungry progress bar (during Stamp loading)
* Verbose debugging options
* (Also, mentioned debugging _at all_, in INSTALL.txt)
Bumping version to 0.9.23a, to match the test build he's working
on for macOS.
