add \nodetitle command to specify titles of nodes in dependency graph

[Hagb]

The plastexdepgraph part is submitted in
PatrickMassot/plastexdepgraph#9.

[Hagb]

An example of bolded main theorem and badly displayed graph caused by long names:

\begin{theorem}
	\label{thm:a-very-long-theorem-or-something-else-label-1}
	\nodetitle{<a-very-long-<BR/>theorem-or-<BR/>something-else-<BR/>label-1>}
\end{theorem}

\begin{theorem}
	\label{thm:a-very-long-theorem-or-something-else-label-2}
	\nodetitle{<a-very-long-<BR/>theorem-or-<BR/>something-else-<BR/>label-2>}
\end{theorem}

\begin{theorem}
	\label{thm:a-very-long-theorem-or-something-else-label-3}
	\nodetitle{<a-very-long-<BR/>theorem-or-<BR/>something-else-<BR/>label-3>}
\end{theorem}

\begin{theorem}
	\label{thm:a-very-long-theorem-or-something-else-label-4}
	\nodetitle{<a-very-long-<BR/>theorem-or-<BR/>something-else-<BR/>label-4>}
\end{theorem}

\begin{theorem}
	\label{thm:a-very-long-theorem-or-something-else-label-5}
	\nodetitle{<a-very-long-<BR/>theorem-or-<BR/>something-else-<BR/>label-5>}
\end{theorem}

\begin{theorem}
	\label{thm:a-very-long-theorem-or-something-else-label-6}
	\nodetitle{<a-very-long-<BR/>theorem-or-<BR/>something-else-<BR/>label-6>}
\end{theorem}

\begin{theorem}
	\label{thm:a-very-long-theorem-or-something-else-label-7}
	\nodetitle{<a-very-long-<BR/>theorem-or-<BR/>something-else-<BR/>label-7>}
\end{theorem}

\begin{theorem}
	\label{thm:an-import-theorem}
	\nodetitle{<<B>an-import-theorem</B>>}
\end{theorem}

without \nodetitle:	with linebreak in \nodetitle

(typo: import -> important)