Add "cascade" filter run prefix and use it to control view templates (#6168)

* Initial Commit

* Set currentTiddler and ..currentTiddler for filter evaulation

* Precompile the filters for performance

* Add explicit test for empty result when no filter passes

* Use the cascade filter run prefix to choose the view template body template

* Use the cascade mechanism to choose between the edit and view templates

* Simplify cascade filter

Thanks @saqimtiaz

* Add control panel UI for inspecting the template cascades

* Refactor import listing and plugin listing as alternate body templates

As suggested by @pmario

* Refer to $:/core/ui/{View|Edit}Template via their associated config tiddlers

* Fix typo in previous commit

* Add demo of custom story tiddler template

* Tweak control panel wording

* Standardise "Story Tiddler Template" nomenclature

* Add a cascade for the editor template body

* Add a cascade for the view template title

* Avoid unwanted whitespace

* Add a cascade for dynamically choosing tiddler icons

core/ui/ViewTemplate/body/code.tid

@@ -0,0 +1,3 @@
+title: $:/core/ui/ViewTemplate/body/code
+
+<$codeblock code={{{ [<currentTiddler>get[text]] }}} language={{{ [<currentTiddler>get[type]else[text/vnd.tiddlywiki]] }}}/>