gstrvbuilder: Add g_strv_builder_take

g_strv_builder_take() allows to transfer ownership of the passed in
string.

This can be useful to avoid additional allocations when using functions
that transfer ownership to the caller like g_strdup_printf().

The testcase uses g_strv_builder_take and g_strv_builder_add to demo
that calls can be mixed.

docs/reference/glib/glib-sections.txt.in

@@ -1604,6 +1604,7 @@ g_strv_builder_unref
 g_strv_builder_add
 g_strv_builder_addv
 g_strv_builder_add_many
+g_strv_builder_take
 g_strv_builder_end
 
 <SUBSECTION>