add is_default script function

doc/function/index.txt

@@ -78,6 +78,7 @@ These functions are built into the program, other [[type:function]]s can be defi
 | [[fun:exclusive_choice]]		Require that at most one of the given choices is selected.
 | [[fun:require_exclusive_choice]]	Require that exactly one of the given choices is selected.
 | [[fun:remove_choice]]			Remove the given choices from a multiple choice value.
+| [[fun:is_default]]			Check if a field is in its default state.
 	
 ! Images			<<<
 | [[fun:linear_blend]]		Blend two images together using a linear gradient.

doc/function/is_default.txt

@@ -0,0 +1,10 @@
+Function: is_default
+
+--Usage--
+> is_default(some_field)
+
+Returns true if the field is in its default state, that is, if the user hasn't modified the value yet, false otherwise.
+
+--Parameters--
+! Parameter	Type			Description
+| @input@	[[type:field]]		Field to test.