Added tagfile generation to doxygen files.

This enables links between modules, by generating a tagfile when module A is generated, and using that tagfile when generating module B.

See also http://stackoverflow.com/questions/8247189/doxygen-is-slow/8247993#8247993

Documentation/Doxygen/Polycode.doxygen

@@ -1524,7 +1524,7 @@ TAGFILES               =
 # When a file name is specified after GENERATE_TAGFILE, doxygen will create 
 # a tag file that is based on the input files it reads.
 
-GENERATE_TAGFILE       = 
+GENERATE_TAGFILE       = tags/Polycode_core.tag
 
 # If the ALLEXTERNALS tag is set to YES all external classes will be listed 
 # in the class index. If set to NO only the inherited external classes 

Documentation/Doxygen/Polycode_ui.doxygen

@@ -1518,7 +1518,9 @@ SKIP_FUNCTION_MACROS   = YES
 # If a tag file is not located in the directory in which doxygen 
 # is run, you must also specify the path to the tagfile here.
 
-TAGFILES               = 
+# Specify the relative location of the core docs from the html/ folder,
+# which is ../../Polycode/html
+TAGFILES               = tags/Polycode_core.tag=../../Polycode/html/
 
 # When a file name is specified after GENERATE_TAGFILE, doxygen will create 
 # a tag file that is based on the input files it reads.