Re-structured ArcLib for the last time

Re-install the ArcLib libraries + extensions, and in the code change

arc_extname

to

arc.extname

for all extensions.

All extensions will now go into the arc/ folder, similar to how Derelict works. Each extension has its own sub-folder. DSSS can install / uninstall extensions as need be, and ArcLib can now be as big or as small as the users want it, allowing them to pick and choose which features to use.