Add -V beameroption variable (#4359)

* LaTeX template: Use `pgfpages` package; this is needed for notes on second screen in beamer
* LaTeX template: Add `beameroption` variable in template

MANUAL.txt

@@ -1468,6 +1468,9 @@ LaTeX variables are used when [creating a PDF].
 :   option for document class, e.g. `oneside`; may be repeated
     for multiple options
 
+`beameroption`
+:   In beamer, add extra beamer option with `\setbeameroption{}`
+
 `geometry`
 :   option for [`geometry`] package, e.g. `margin=1in`;
     may be repeated for multiple options

data/templates/default.latex

@@ -7,10 +7,14 @@ $endif$$if(dir)$$if(latex-dir-rtl)$
 $endif$$endif$%
 \documentclass[$if(fontsize)$$fontsize$,$endif$$if(lang)$$babel-lang$,$endif$$if(papersize)$$papersize$paper,$endif$$if(beamer)$ignorenonframetext,$if(handout)$handout,$endif$$if(aspectratio)$aspectratio=$aspectratio$,$endif$$endif$$for(classoption)$$classoption$$sep$,$endfor$]{$documentclass$}
 $if(beamer)$
+\usepackage{pgfpages}
 \setbeamertemplate{caption}[numbered]
 \setbeamertemplate{caption label separator}{: }
 \setbeamercolor{caption name}{fg=normal text.fg}
 \beamertemplatenavigationsymbols$if(navigation)$$navigation$$else$empty$endif$
+$for(beameroption)$
+\setbeameroption{$beameroption$}
+$endfor$
 $endif$
 $if(beamerarticle)$
 \usepackage{beamerarticle} % needs to be loaded first