GAction: add function for printing detailed names

A counterpart for parsing of detailed actions into (name, target) pairs,
this new function prints them back.

We also add a new function to check for validity of action names.  Only
valid action names are allowed when printing.  Parsing accepts _some_
invalid names for backwards compatibility.

https://bugzilla.gnome.org/show_bug.cgi?id=704157

docs/reference/gio/gio-sections.txt

@@ -3107,6 +3107,7 @@ GAction
 GActionInterface
 
 <SUBSECTION>
+g_action_name_is_valid
 g_action_get_name
 g_action_get_parameter_type
 g_action_get_state_type
@@ -3122,6 +3123,7 @@ g_action_activate
 
 <SUBSECTION>
 g_action_parse_detailed_name
+g_action_print_detailed_name
 
 <SUBSECTION Standard>
 g_action_get_type