licenses: convert license tags to SPDX

jedit: go to end of file if there is no error

(idea by Dominique Unruh)

misc: goto-error jEdit macro: handle errors, fix string compare

When updating to Isabelle 2017 the non-error case was neglected and gave
a stack dump rather than a sane message. This has now been addressed.

Apparently the Beanshell scripting environment in jEdit prior to 5.5.0
accepted '==' as string comparison. In 5.5.0 it is no longer accepted,
and requires .equals() as in normal Java.

Isabelle2017: update Isabelle/jEdit goto-error macro for RC0

jedit goto-error macro doc: installation path can be subdir

Since writing this, we have discovered that macros can live in arbitrary
hierarchies inside the macros folder, e.g. jedit/macros/Isabelle/, which
then show up in the macros menu. Very nice. Installation instructions
updated.

JEdit macro: goto-error (first version)

Jumps to first error in current theory file (presumably the currently
focused textArea).

The buffer overview indicates where the error is, but it's hard to go
directly to the offending command, and in large files the overview
buffer doesn't cover the whole file anyway.