filesystem MAINTENANCE move public ly_ctx* functions into context.h

Private definition of the ly_ctx structure is not in common.h header
file and all the context's public functions will be declared in
context.h.
diff --git a/Doxyfile.in b/Doxyfile.in
index 6d7170a..dbc9674 100644
--- a/Doxyfile.in
+++ b/Doxyfile.in
@@ -782,6 +782,7 @@
 # Note: If this tag is empty the current directory is searched.
 
 INPUT                  = ./src/libyang.h \
+			 ./src/context.h \
                          ./src/tree_schema.h \
                          ./src/log.h \
                          ./src/set.h \