Use flexbox for vertical tabs

Thus stepping into a new world:

http://caniuse.com/flexbox

The vertical tabs are now completely reusable, with sensible wrapping
behaviour.
Add "before" and "after" filter operators

Fixes #357 by adding new “before” and “after” filter operators.
Remove obsolete filter operators docs

It was overwritten by a later version in editions/tw5.com/filters.
Update release note
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
Update reveal widget docs
Fix dropdown behaviour in advanced settings
fr-FR translation of Advanced Settings in the ControlPanel
edits to the fr-FR translation for the Edit Template
Merge pull request #609 from xcazin/fr-FR

fr-FR translations for new strings in the Control Panel, plus edits to an older one
Fix problem with widget.getStateQualifier()

Fixed to take into account the recent change to read variables from the
parent widget: e60fc9f81f2c8f0d115543d8d330a1d68f9b890a
Fix line height
Update release note
Fix tab button padding

Lack of padding caused a visual glitch in Firefox
Add chinese translations of Advanced Settings in the ControlPanel
Get rid of the tweakParseTreeNode() hack

It’s an embarrassing hangover from a refactoring of the parsing
mechanism last year.
Tabs appearance tweaks