refs #1433: Rewrite file caching

Increase cache size and make it thread-safe by usage of
 * uthash (https://github.com/troydhanson/uthash/blob/master/src/uthash.h) and
 * gconstructor (https://github.com/winlibs/glib/blob/master/glib/gconstructor.h)
headers.

Also many cosmetic changes (idents and line breaks) were added to make code more readable.

Test case ModelicaTest.Utilities.Streams was extended to consider multiple open files.

git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@7813 7ce873d0-865f-4ce7-a662-4bb36ea78beb