Some external passage follow-ups

* Added a mauve emblem denoting that the given passage contains links to
StoryIncludes passages.
* Links to known StoryIncludes passages are now syntax-highlit mauve.
* Altered a number of findWidget() calls to become passageExists()
calls.
* Removed the testdata folder, as it was going unused.
* Fixed a bug where including passages from a .twee file would fail due
to them lacking a pos.
* Fixed the version numbers in install.nsi.