session_label: make typedef String public

Internally, Genode::Session_label has a typedef from String<capacity()> to
String. To have this typedef public is especially useful when reading a
label from an XML node. This can then be written as

! Session_label label = node.attribute_value("label", Session_label::String());

instead of

! Session_label label = node.attribute_value("label", String<160>());

which would be less generic (Session_label cannot be used directly as there
is no appropriate ascii_to implementation).

Issue #2840

repos/base/include/base/session_label.h

@@ -25,13 +25,12 @@ struct Genode::Session_label : String<160>
 {
 	private:
 
-		typedef String<capacity()> String;
-
 		static char const *_separator()     { return " -> "; }
 		static size_t      _separator_len() { return 4; }
 
 	public:
 
+		using String = String<capacity()>;
 		using String::String;
 
 		Session_label last_element() const