LaTeX writer: Fix regression in table header position.

In recent versions the table headers were no longer bottom-aligned
(if more than one line).  This patch fixes that by using minipages
for table headers in non-simple tables.

Closes #7347.

test/command/5367.md

@@ -24,11 +24,15 @@ hello\footnote{doc footnote}
   >{\centering\arraybackslash}p{(\columnwidth - 0\tabcolsep) * \real{0.17}}@{}}
 \caption[Sample table.]{Sample table.\footnote{caption footnote}}\tabularnewline
 \toprule
-Fruit\footnote{header footnote} \\
+\begin{minipage}[b]{\linewidth}\centering
+Fruit\footnote{header footnote}
+\end{minipage} \\
 \midrule
 \endfirsthead
 \toprule
-Fruit{} \\
+\begin{minipage}[b]{\linewidth}\centering
+Fruit{}
+\end{minipage} \\
 \midrule
 \endhead
 Bans\footnote{table cell footnote} \\