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.
Magic tool documentation now split into separate files, and referenced
(as a directory) from README, so that users can find docs to any additional
tools (ones not included by default with Tux Paint) that are installed.
Added new --datadir option, to separate path to brushes/stamps/etc. from that of saved files.
Improved docs on where savedir default is.
Made sure --help, man tuxpaint, and OPTIONS docs all covered all command-line options.
Noted SDL_Pango makes locale-specific fonts unnecessary.
Added "--plugindocprefix" option to tp-magic-config, for where docs should go.
Improved plugin API documentation.
Improved layout of man pages a little.
