[isabelle-dev] isabelle components

[Makarius]

Various changes leading up to Isabelle/ae7429d66b1e attempt to disentangle 
the component situation on the Isabelle repository, as opposed to regular 
releases.

See the end of README_REPOSITORY and the system manual on "Add-on 
components" and the "isabelle components" tool.

Short version:

   * init_components (bash function) admits explicit directory
     specifications, instead of hardwired $COMPONENT from before;

   * the underlying init_component (bash function) is liberal about missing
     components, but marks them explicitlly;

    * "isabelle components" (command line tool) can be used to resolve
     missing components from a local or remote repository; downloads
     are locally cached in the target directory;

    * Admin/components/ now has various symbolic groups of components, most
     notably "main";

I will later add components/polyml and components/smlnj to point to 
recent versions of the same with certain default settings -- this is going 
to be seperate to allow easy shuffling of several ML systems, or doing 
settings by hand.


Eventually, it should become immediately clear for the 2-3 maintainers of 
Isabelle components where to put things, and the consumers of the same 
where to get them.  (So far I am still discovering new corners where I 
have to place another copy of jdk-7u6 from yesterday.)


The intermediate Admin/init_components script is still there for a few 
more days, to facilitate migration of local settings.


 	Makarius