Add documentation

- Add file documentation to about 1/4 of the files in Source
- Copy over a lot of the documentation from the sanctuary/notes repo
- Standardise all the existing documentation
- Create a configuration for Doxygen
- Add more documentation (engine.cpp is now fully documented)

doxygen.config

@@ -0,0 +1,34 @@
+PROJECT_NAME           = "Devilution"
+PROJECT_BRIEF          = "Diablo devolved - magic behind the 1996 computer game"
+OUTPUT_DIRECTORY       = docs
+INPUT                  = ./ ./Source
+INPUT_ENCODING         = UTF-8
+FILE_PATTERNS          = *.c \
+                         *.cpp \
+                         *.h \
+                         *.inc
+GENERATE_LATEX         = NO
+
+# Call graphces alows logical navigation of the docs
+CALL_GRAPH             = YES
+CALLER_GRAPH           = YES
+
+# all.h includes everything and everything inclues it so the graph is usless past depth 1
+MAX_DOT_GRAPH_DEPTH    = 1
+
+SOURCE_BROWSER         = YES
+EXTRACT_STATIC         = YES
+JAVADOC_AUTOBRIEF      = YES
+OPTIMIZE_OUTPUT_FOR_C  = YES
+
+HTML_COLORSTYLE_HUE    = 0
+HTML_COLORSTYLE_SAT    = 125
+HTML_COLORSTYLE_GAMMA  = 200
+HTML_DYNAMIC_SECTIONS  = YES
+
+DOT_IMAGE_FORMAT       = svg
+INTERACTIVE_SVG        = YES
+
+# Not all files are documented yet
+EXTRACT_ALL            = YES
+