Re-jigger some of the plugins code to make it more useful:

Re-jigger some of the plugins code to make it more useful:

- Allow plugins to be targetted to a platform, so we can ship multi-platform config files.
- Allow path elements (BIN dir, working dir, PG bin dir, EDB bin dir etc) to be substituted into the command line and keyfile.
- Move utilities.ini into the main share directory and rename to plugins.ini.
- Allow plugins to be targetted to specific server types.
- Ensure the default plugin is currently valid before executing it.

git-svn-id: svn://svn.pgadmin.org/trunk/pgadmin3@7651 a7884b65-44f6-0310-8a51-81a127f17b15