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.
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