Cleaned up collision systems and gave them general includes, reducing duplicate code.
Cleaned up graphics system and windowing system bridges to make use of common
includes reducing duplicate code.
Moved SFML audio functions to enigma_user namespace and added string include
to ASbasic.h
Merged common widget system headers together into general includes, and fixed
a bug with GTK not compiling because show_error was not defined, added it with
a little //TODO: statement.
Merge pull request #261 from RobertBColton/master

Cleaned up more systems for general includes