Support for PDF 2.0+

[rausm]

In case it's not obvious, for now PDF with version 2.0 will not work, as iText uses just last char/digit from the version, and so the PDF is considered 1.0. Since we won't need to update past PDF-1.7, it should suffice until something other breaks ;-)

NB:

• Hand-picked changes from my version (which I forked around 1.5.1 and, sadly, let diverge; contains a few changes no-one would ever want, so "just sending all the changes" was not possible).
• Had to patch iText a bit (to get at the version number)
• I hate Java with a vengence and only use it in direst cases of self-defence, it might show.
• Haven't tried to compile "the standard way" (I use IntelliJ "Build Artifact" to compile mine version into one giantic .jar).
• Hopefully should "just work"(tm).

PS: I also integrated (into my version) IcePdf for preview (manages to correctly preview a lot of PDF's, SUPPORTS ZOOMED rendering !!! --> No more wasting of gigs of memory to preview large pdf in a 100x100px square; was the only option to get it working on 32bit JVMs).

If there's interest, let me know and I'll try to pull relevant changes out.

[rausm]

Looking at it one last time: will require checking/updating either the
LOGGER.info(RES.get("console.updateVersion", new String[] { pdfVer, hashVer }));
or the "console.updateVersion" resource..

[rausm]

I improved upon the original PR a bit. At the time of posting, I just hand-transplanted the code in plain text editor (I wasn't sure when I'll have free time, and was putting off sending my changes upstream few years already ...).

Now I imported the official/your version into InteliJ, fixed a few mishaps (in mine fixes, of course ;-) and verified that it builds.
Let me know if anything else need improving, or if I should squash the changes into one commit (ditto for the drag&drop PR).

[kwart]

@rausm Sorry for such a late reply. Could you rebase this for the latest master?