Fix problem with variables containing an empty string

See
https://github.com/Jermolene/TiddlyWiki5/commit/e60fc9f81f2c8f0d115543d8
d330a1d68f9b890a#commitcomment-6301921
Clean up whitespace
Extend .tid files to allow single line text fields

To make it easier to create tiddlers that don’t include a terminating
newline in their text
Make permalink behaviour configurable

In the process introducing a new advanced settings tab

Fixes #600