* $let widget added and tested
* Documentation for $let, doc improvements for $vars
* let properly avoids refreshing when possible
* $let Changes as recommended by others
* Removed superfluous super method call
Also improved $let test
* add EventListeners in the render() method instead of the constructor
* scrollable widget: add EventListeners in render() method instead of constructor +
... move logic from constructor to render()
* linkcatcher: add EventListeners in render() instead of constructor
* fieldmangler: add EventListeners in render() instead of constructor
* edit-bitmap: initialise editorOperations in render() instead of constructor
* list-widget: initialise storyviews in render() instead of constructor
* vars widget: execute Widget.call(this) in render() instead of constructor
... not shure what this should do
* Update fieldmangler.js
* Update edit-bitmap.js
* Update linkcatcher.js
* Update navigator.js
* Update scrollable.js
* Update list.js
* Update vars.js
It thereby reduces code complexity that would arise when setting
many variables using "<$set>".
```
\define helloworld() Hello world!
<$vars greeting="Hi" me={{!!title}} sentence=<<helloworld>>>
<<greeting>>! I am <<me>> and I say: <<sentence>>
</$vars>
```
How this Widget differs from the set widget:
* Variables may be created by using the "key=value" notation
that you already know from widgets like action-setfield.
* You cannot specify a fallback ("emptyValue")
* You cannot use a filter to produce a conditional variable assignement
Original discussion that led to the creation of this widget:
https://github.com/Jermolene/TiddlyWiki5/issues/1610
