Wire keyboard into save naming, add fatal error overlay, cutscene keyboard item

- uimainmenu: removed the temporary keyboard test entry now that the
  keyboard has a real caller.
- uiselectsave: picking an empty save slot now opens the keyboard to
  name it (localized title "Enter game save file name"), writes the
  named slot to disk (mirroring the existing delete-confirm write
  pattern), and on a write failure opens the new fatal error overlay.
- uikeyboardopen_t gains a title override (NULL keeps the default
  "ENTER YOUR TEXT").
- New ui/overlay/uifatalerror: a minimal fullscreen, non-dismissible
  overlay (message + QUIT button, back disabled) for unrecoverable
  errors - a nicer alternative to a raw assert crash. Draws above
  everything else in the UI element list.
- New CUTSCENE_ITEM_TYPE_KEYBOARD cutscene item: opens the keyboard
  with an embedded uikeyboardopen_t, blocks the cutscene until it
  closes, then caches the entered text via a new general-purpose
  cutscenesystem_t.textCache (CUTSCENE_TEXT_CACHE_MAX, with get/set
  accessors) mirroring the existing entity/area/text-mini id caching
  pattern.

assets/locale/en_US.po

@@ -82,6 +82,9 @@ msgstr "Delete a Save"
 msgid "ui.select_save.delete_confirm"
 msgstr "Are you sure you want to delete this save?"
 
+msgid "ui.select_save.name_title"
+msgstr "Enter game save file name"
+
 msgid "ui.save_slot.number_format"
 msgstr "Slot %d"