Make PDFTextStreamEngine class public

In 1.8 branch text extraction was performed by monolithic PDFTextStripper class. In 2.0 it was refactored in PDFTextStreamEngine and new PDFTextStripper classes which extends the former. The new PDFTextStripper contains a lot of new functionality that is not always needed by the clients extending it (page and bookmark position state bookkeeping, writer procedures). To avoid the burden of maintaining unneeded functionality of this class by the clients and also to save the run time there's a sense for them to extend PDFTextStreamEngine directly. Currently, PDFTextStreamEngine class is not declared public. This makes it package-private and does not let the clients from other packages to extend it.
Suggestion: declare PDFTextStreamEngine class and it's constructor as public.