gobject: Add g_{param_spec,signal}_is_valid_name() functions

Making this validation code public allows projects to validate a
GParamSpec name before creating it. While hard-coded GParamSpec don't
need this, we can't afford crashing the main program for dynamically
generated GParamSpec from user-created data.

In such case, we will need to validate the param names and return errors
instead of trying to create a GParamSpec with invalid names.

Includes modifications from Philip Withnall and Emmanuele Bassi to
rearrange the new function addition and split it into one function for
GParamSpecs and one for GSignals.

gobject/gparam.h

@@ -395,6 +395,9 @@ GLIB_AVAILABLE_IN_ALL
 GType	g_param_type_register_static	(const gchar		  *name,
 					 const GParamSpecTypeInfo *pspec_info);
 
+GLIB_AVAILABLE_IN_2_66
+gboolean g_param_spec_is_valid_name    (const gchar              *name);
+
 /* For registering builting types */
 GType  _g_param_type_register_static_constant (const gchar              *name,
 					       const GParamSpecTypeInfo *pspec_info,