Use beamertemplate for footer

Added possibility to add a custom footer. Usage is shown in the demo slide deck.

demo/demo.tex

@@ -17,7 +17,7 @@
 \date{\today}
 \author{Matthias Vogelgesang}
 \institute{Center for modern beamer themes}
-% \titlegraphic{\hfill\includegraphics[height=1.5cm]{logo/logo}}
+\titlegraphic{\hfill\includegraphics[height=1.5cm]{logo.pdf}}
 
 \begin{document}
 
@@ -66,7 +66,7 @@
 \end{frame}
 
 {
-\metroset{titleformat frame=smallcaps}
+    \metroset{titleformat frame=smallcaps}
 \begin{frame}{Small caps}
 	This frame uses the \texttt{smallcaps} titleformat.
 
@@ -281,6 +281,14 @@ or show \textbf{bold} results.\end{verbatim}
   \end{quote}
 \end{frame}
 
+{%
+\setbeamertemplate{frame footer}{My custom footer}
+\begin{frame}[fragile]{Frame footer}
+    \themename defines a custom beamer template to add a text to the footer. It can be set via
+    \begin{verbatim}\setbeamertemplate{frame footer}{My custom footer}\end{verbatim}
+\end{frame}
+}
+
 \begin{frame}{References}
   Some references to showcase [allowframebreaks] \cite{knuth92,ConcreteMath,Simpson,Er01,greenwade93}
 \end{frame}

source/beamerouterthememetropolis.dtx

@@ -116,6 +116,11 @@
 %    Templates for the frame number. Can be omitted, shown or displayed as a
 %    fraction of the total frames.
 %    \begin{macrocode}
+\defbeamertemplate{frame footer}{none}{}
+\defbeamertemplate{frame footer}{custom}[1]{ #1 }
+%    \end{macrocode}
+%
+%    \begin{macrocode}
 \defbeamertemplate{frame numbering}{none}{}
 \defbeamertemplate{frame numbering}{counter}{\insertframenumber}
 \defbeamertemplate{frame numbering}{fraction}{
@@ -131,8 +136,9 @@
 \defbeamertemplate{headline}{plain}{}
 \defbeamertemplate{footline}{plain}{%
   \begin{beamercolorbox}[wd=\textwidth, sep=3ex]{footline}%
-    \hfill%
     \usebeamerfont{page number in head/foot}%
+    \usebeamertemplate*{frame footer}
+    \hfill%
     \usebeamertemplate*{frame numbering}
   \end{beamercolorbox}%
 }