Improve/fix handling of XML Catalogs

[matthiasblaesing]

NetBeans allows users to validate XML files against their grammars. XML Catalogs allow users to supply their own grammars and enable faster access to the DTD and schema files. NetBeans has to options:

• Users can register their DTDs and XSDs in the "User Catalog"
• Users can register XML catalog files (OASIS XML catalog, XCatalog, XMLCatalog)

There are multiple problems here:

1. The registered XML catalog file entries were not saved and thus only usable in a single session
2. The default format for XML catalog files is the OASIS XML catalog format, but this was listed as second option only
3. Registering a catalog did not give feedback, whether the catalog seemed to be correct

With this change the problems are solved:

After a restart registered catalog is still present (see the "Resolver at ..." entry):

The first entry of the provider list is now the OASIS Catalog Resolver:

If the catalog does not yield entries, this is shown:

Closes: #8464

[lkishalmi]

Well, looks good.

However while being here, the files in impl could use a bit more facelift, removing pre Java 1.2 collections and add a few use of generics.

[matthiasblaesing]

Thanks for review. Will merge.