Support a list of images for titlegraphic (#10246)

Allow a list of title graphics in default.beamer
Title graphic options will be applied to each title graphic.
Images will be separated by `\enspace`.

MANUAL.txt

@@ -2902,7 +2902,7 @@ These variables change the appearance of PDF slides using [`beamer`].
 :   options for LaTeX beamer themes (lists)
 
 `titlegraphic`
-:   image for title slide
+:   image for title slide: can be a list
 
 `titlegraphicoptions`
 :   options for title slide image

data/templates/default.beamer

@@ -111,7 +111,10 @@ $if(institute)$
 \institute{$for(institute)$$institute$$sep$ \and $endfor$}
 $endif$
 $if(titlegraphic)$
-\titlegraphic{\includegraphics$if(titlegraphicoptions)$[$for(titlegraphicoptions)$$titlegraphicoptions$$sep$, $endfor$]$endif${$titlegraphic$}}
+\titlegraphic{
+$for(titlegraphic)$
+  \includegraphics$if(titlegraphicoptions)$[$for(titlegraphicoptions)$$titlegraphicoptions$$sep$, $endfor$]$endif${$titlegraphic$}$sep$\enspace
+$endfor$}
 $endif$
 $if(logo)$
 \logo{\includegraphics{$logo$}}