add connection_get_jid()

Use a singleton `Jid` inside the connection instead of always re-creating
a `Jid` from the same string.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

src/tools/editor.c

@@ -56,7 +56,7 @@ get_message_from_editor(gchar* message, gchar** returned_message)
 
     auto_gchar gchar* filename = NULL;
     GError* glib_error = NULL;
-    auto_char char* jid = connection_get_barejid();
+    const char* jid = connection_get_barejid();
     if (jid) {
         filename = files_file_in_account_data_path(DIR_EDITOR, jid, "compose.md");
     } else {