lisp/ox-beamer.el: add onlyenv to `org-beamer-environments-default'

* lisp/ox-beamer.el: (org-beamer-environments-default): Add "onlyenv"
as suggested long time ago in the mailing list or by
https://tippenhauer.de/post/writing-presentations-in-org-mode-markup/

TINYCHANGE

diff --git a/lisp/ox-beamer.el b/lisp/ox-beamer.el
index 527bb55e5..433d98445 100644
--- a/lisp/ox-beamer.el
+++ b/lisp/ox-beamer.el
@@ -198,7 +198,8 @@ through `org-beamer-environments-extra' variable.")
     ("example"        "e" "\\begin{example}%a[%h]%l"        "\\end{example}")
     ("exampleblock"   "E" "\\begin{exampleblock}%a{%h}%l"   "\\end{exampleblock}")
     ("proof"          "p" "\\begin{proof}%a[%h]"          "\\end{proof}")
-    ("beamercolorbox" "o" "\\begin{beamercolorbox}%o{%h}" "\\end{beamercolorbox}"))
+    ("beamercolorbox" "o" "\\begin{beamercolorbox}%o{%h}" "\\end{beamercolorbox}")
+    ("onlyenv"        "O" "\\begin{onlyenv}%a"            "\\end{onlyenv}"))
   "Environments triggered by properties in Beamer export.
 These are the defaults - for user definitions, see
 `org-beamer-environments-extra'.")