Moved some beamer code in default.latex template.

This change allows beamer themes to change the
template and font (as Metropolis does).

Closes #4450.

data/templates/default.latex

@@ -20,6 +20,41 @@ $endif$
 $for(beameroption)$
 \setbeameroption{$beameroption$}
 $endfor$
+% Prevent slide breaks in the middle of a paragraph:
+\widowpenalties 1 10000
+\raggedbottom
+$if(section-titles)$
+\setbeamertemplate{part page}{
+\centering
+\begin{beamercolorbox}[sep=16pt,center]{part title}
+  \usebeamerfont{part title}\insertpart\par
+\end{beamercolorbox}
+}
+\setbeamertemplate{section page}{
+\centering
+\begin{beamercolorbox}[sep=12pt,center]{part title}
+  \usebeamerfont{section title}\insertsection\par
+\end{beamercolorbox}
+}
+\setbeamertemplate{subsection page}{
+\centering
+\begin{beamercolorbox}[sep=8pt,center]{part title}
+  \usebeamerfont{subsection title}\insertsubsection\par
+\end{beamercolorbox}
+}
+\AtBeginPart{
+  \frame{\partpage}
+}
+\AtBeginSection{
+  \ifbibliography
+  \else
+    \frame{\sectionpage}
+  \fi
+}
+\AtBeginSubsection{
+  \frame{\subsectionpage}
+}
+$endif$
 $endif$
 $if(beamerarticle)$
 \usepackage{beamerarticle} % needs to be loaded first
@@ -198,43 +233,6 @@ $if(graphics)$
 % using explicit options in \includegraphics[width, height, ...]{}
 \setkeys{Gin}{width=\maxwidth,height=\maxheight,keepaspectratio}
 $endif$
-$if(beamer)$
-% Prevent slide breaks in the middle of a paragraph:
-\widowpenalties 1 10000
-\raggedbottom
-$if(section-titles)$
-\setbeamertemplate{part page}{
-\centering
-\begin{beamercolorbox}[sep=16pt,center]{part title}
-  \usebeamerfont{part title}\insertpart\par
-\end{beamercolorbox}
-}
-\setbeamertemplate{section page}{
-\centering
-\begin{beamercolorbox}[sep=12pt,center]{part title}
-  \usebeamerfont{section title}\insertsection\par
-\end{beamercolorbox}
-}
-\setbeamertemplate{subsection page}{
-\centering
-\begin{beamercolorbox}[sep=8pt,center]{part title}
-  \usebeamerfont{subsection title}\insertsubsection\par
-\end{beamercolorbox}
-}
-\AtBeginPart{
-  \frame{\partpage}
-}
-\AtBeginSection{
-  \ifbibliography
-  \else
-    \frame{\sectionpage}
-  \fi
-}
-\AtBeginSubsection{
-  \frame{\subsectionpage}
-}
-$endif$
-$endif$
 $if(links-as-notes)$
 % Make links footnotes instead of hotlinks:
 \DeclareRobustCommand{\href}[2]{#2\footnote{\url{#1}}}