Fix compilation of fr/beamer.lyx with XeTeX

The compilation did not succeed before because of an é in an ERT
inset. This is changed to \'{e}.

Also correct spelling of "decouvert" to "découvert".

Alternatives would be to use a LyX note or a comment inset. Since
the original version uses ERT, I do not change that.

This commit fixes the following ctest:

  export/examples/fr/beamer-article_pdf4_texF

It likely also fixes the corresponding test for fr/beamer.lyx, but
since the default output format is set to pdf2 we do not run the
test.

For more information, see:
https://www.mail-archive.com/search?l=mid&q=20160919192413.ftnz4gk6dl5w3k5c%40Opti1604

(cherry picked from commit 9f3518bcd2)

lib/examples/fr/beamer.lyx

@@ -66,11 +66,12 @@
 \papercolumns 1
 \papersides 1
 \paperpagestyle default
-\tracking_changes false
+\tracking_changes true
 \output_changes false
 \html_math_output 0
 \html_css_as_file 0
 \html_be_strict false
+\author 274215730 "scott"
 \end_header
 
 \begin_body
@@ -2726,7 +2727,23 @@ status collapsed
 
 \begin_layout Plain Layout
 
-%Pour illustrer le différence entre decouvert et visible:
+%Pour illustrer le diff
+\change_deleted 274215730 1474337976
+é
+\change_inserted 274215730 1474337982
+
+\backslash
+'{e}
+\change_unchanged
+rence entre d
+\change_deleted 274215730 1474337993
+e
+\change_inserted 274215730 1474337996
+
+\backslash
+'{e}
+\change_unchanged
+couvert et visible:
 \end_layout
 
 \begin_layout Plain Layout