Reduce list title glitches when switching between menus

Each time viewportmanager_theme_enable (or _undo) is called,
the SBS title is reset, even if the theme remains enabled.
Thus switching from one menu to another, if do_menu is called
again, briefly results in an empty title before the correct
one is displayed. Even unchanged titles will unnecessarily
flash for a moment. Other theme elements that are drawn using
conditions based on the title, may also appear glitchy.

This patch adds a way to make the status bar title persist by
copying it to a static buffer. Persistent titles are not reset
by toggle_theme (although scrolling will be stopped in
viewportmanager_theme_undo), so that the theme can immediately
display the appropriate title in do_menu, simplelist_show_list,
the yesno screen, or plugins that want to keep the theme enabled.

Change-Id: I1ec8f233b730321793eb7d3cad51496ee1b35440

apps/plugin.c

@@ -874,6 +874,7 @@ static const struct plugin_api rockbox_api = {
 
     /* new stuff at the end, sort into place next time
        the API gets incompatible */
+    sb_set_persistent_title,
 };
 
 static int plugin_buffer_handle;