Created Mirror and Flip Magic tools as plugins (except sound); hooks
added to Magic plugin API to allow starter and starter-undo mirror/flip.
Created Fade and Darken Magic tools as plugins.
Moved magic tool sounds to 'magic'.
Added flip and mirror icons to main Tux Paint UI (used to share magic tools')
Created 'tp-magic-config.sh' to get CFLAGS for compiling magic tool plugins.
Improved magic tool plugin API.
Committed some documentation on magic tool plugin API.
