- Use identical logic for looking up <dependency>s and <command>s.
- Remove duplicate (but inconsistent and incomplete) logic from
  script.cpp that was used to search for the <command> again
- Remove <check> element from .inx format
  It seems unused (at the very least by core extensions) and
  redundant to <dependency> checking
- Deprecate the <command>-specific "reldir" attribute.
  Consistently use the functionally identical "location" attribute
  that was only used for <dependency>s before
- Introduce the new relative location value location="inx",
  which looks up <dependencies> and <command>s relative to the
  .inx file's location.

- add license headers to everything
- convert a few files from public domain or LGPL2.1+ to GPL2+
- some archaeology to clarify which files are from which library

This adds the override specifier on all methods which override a virtual
method, whether they were already virtual or missing this specifier.

This turned out to be a bit of a mess due to the interplay between
the classes Script / ExecutionEnv / Effect / PrefDialog.
(Basically they don't talk to each other much but all want to
 contribute to the GUI)

Likely "Script" (and possibly "ExecutionEnv") should be refactored
to let the other classes handle UI exclusively and throw errors
where suitable.

Fixed bugs:
  - https://launchpad.net/bugs/1558160

(bzr r14727)

Fixed bugs:
  - https://launchpad.net/bugs/967416

(bzr r14060)

(bzr r13341.1.145)

Fixed bugs:
  - https://launchpad.net/bugs/1122816

(bzr r12145)

(bzr r11017)