dict CHANGE split dictionary API to private and public part

Make part of the dictionary API public and usable by library users.
It should allow more effective way of memory usage.
diff --git a/Doxyfile.in b/Doxyfile.in
index b683b94..608ee4b 100644
--- a/Doxyfile.in
+++ b/Doxyfile.in
@@ -766,7 +766,8 @@
 INPUT                  = ./src/libyang.h \
                          ./src/tree_data.h \
                          ./src/tree_schema.h \
-                         ./src/xml.h
+                         ./src/xml.h \
+                         ./src/dict.h
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses