Support short title, author, etc. in Beamer (#10244)

+ Add `shorttitle`, `shortsubtitle`, `shortauthor`, `shortinstitute`, `shortdate`
   to default.beamer.
+ Note that the brackets are inside conditionals, because some older beamer
    versions don't allow the empty optional argument.

Closes #10248.

MANUAL.txt

@@ -2907,6 +2907,10 @@ These variables change the appearance of PDF slides using [`beamer`].
 `titlegraphicoptions`
 :   options for title slide image
 
+`shorttitle`, `shortsubtitle`, `shortauthor`, `shortinstitute`, `shortdate`
+:   some beamer themes use short versions of the title, subtitle, author,
+    institute, date
+
 ### Variables for PowerPoint
 
 These variables control the visual aspects of a slide show that

data/templates/default.beamer

@@ -107,15 +107,15 @@ $after-header-includes.latex()$
 $hypersetup.latex()$
 
 $if(title)$
-\title{$title$$if(thanks)$\thanks{$thanks$}$endif$}
+\title$if(shorttitle)$[$shorttitle$]$endif${$title$$if(thanks)$\thanks{$thanks$}$endif$}
 $endif$
 $if(subtitle)$
-\subtitle{$subtitle$}
+\subtitle$if(shortsubtitle)$[$shortsubtitle$]$endif${$subtitle$}
 $endif$
-\author{$for(author)$$author$$sep$ \and $endfor$}
-\date{$date$}
+\author$if(shortauthor)$[$shortauthor$]$endif${$for(author)$$author$$sep$ \and $endfor$}
+\date$if(shortdate)$[$shortdate$]$endif${$date$}
 $if(institute)$
-\institute{$for(institute)$$institute$$sep$ \and $endfor$}
+\institute$if(shortinstitute)$[$shortinstitute$]$endif${$for(institute)$$institute$$sep$ \and $endfor$}
 $endif$
 $if(titlegraphic)$
 \titlegraphic{