Add an official way for users to define custom help viewers

[fingolfin]

Inspired by an email by Bill Allombert to the GAP mailing list:

Right now, we provide a fixed list of help viewers for users. In principle, this list canbe extended by users, but this is undocumented. For example. this is the definition for the "chrome" help viewer:

   # html version with chrome
   HELP_VIEWER_INFO.chrome := rec(
   type := "url",
   show := function(url)
     Exec(Concatenation("chromium-browser \"file://", url,"\" >/dev/null 2>&1 &"));
   end
   );
``

But there really should be a documented way for users to add help viewers. We could just document what is there. But perhaps we can do better?

E.g. assuming that
many (?) browsers can just be started via "COMMANDNAME URL", one could add something like
```gap
BindGlobal("DefineBrowserHelpViewer", function(name, exec)
   HELP_VIEWER_INFO.(name) := rec(
   type := "url",
   show := function(url)
     Exec(Concatenation(exec, " \"file://", url,"\" >/dev/null 2>&1 &"));
   end
   );
end);

A slightly more elaborate version would (optionally) allow adding a placeholder where the URL should be placed; e.g. if we use %, then a user could specify mycommand % --some-arg-that-must-come-after-URLs.

More things that should be improved:

• we could use Process instead of Exec ; that avoids troubles with shell quoting of arguments
  • however, we then also need a way to background such a process
• it's weird that the argument is called "url" but apparently doesn't receive an URL, just a file path. That should be changed.
• perhaps we could filter the list of offered help viewers, by adding a test to each that checks whether it is actually available
• there are other types of help viewers, ideally we'd also offer nicer interfaces for those? From helpview.gi:

##  "text":   a format as allowed by `Pager' (usually rec(lines:=text,
##            start:=linenr, formatted:=true))
##  
##  "url":    an URL (usually local `file://' URL)
##  
##  "dvi":    a filename or a record rec(file:=filename, page:=pagenr)
##            where pagenr is the first page to show
##  
##  "pdf":    same as for "dvi"

A somewhat related issue is #4272.