Use JNLP FileOpenService to access local files

This removes the requirement of signing the Applet JAR. In addition, the
user does not need to trust the applet, because the applet will
specifically ask for each file it wants to access, unless the user chooses
not to be asked again.

This requires loading the applet via jnlp_href parameter, which needs a
JRE6u10 or later version of the Java plugin, but a (signed) old version as
a fallback applet can be provided if desired.

Note that some versions of the Applet Plugin do not support LiveConnect
calls on the HTML applet tag, therefore this applet calls a JavaScript
function setDigestApplet and passes a reference to itself, which can be
stored in a JavaScript variable and used later for calling the applet.
Merge pull request #1 from schierlm/file-open-service

Use JNLP FileOpenService to access local files; preparing to merge this repo into central-search