Restore +popup-ttl

Accidentally removed in a recent commit.

Fixes #692

modules/ui/popup/config.el

@@ -94,6 +94,10 @@ a brief description of some native window parameters that Emacs uses:
     (no-other-window . t))
   "The default window parameters.")
 
+(defvar +popup-ttl 5
+  "The default time-to-live for transient buffers whose popup buffers have been
+deleted.")
+
 (defvar +popup-margin-width 1
   "Size of the margins to give popup windows. Set this to nil to disable margin
 adjustment.")