Add plugin-priority fields for core plugins (#3113)

* Bug in plugin Ordering

Comparing plugin A without a `plugin-priority` field to plugin B with a `plugin-priority= 1` results in the plugins being ordered B, A when it seems like they should be ordered A, B.

* Switch to setting a plugin-priority on the built in themes to allow overriding parts of them.

themes/tiddlywiki/snowwhite/plugin.info

@@ -5,5 +5,6 @@
 	"core-version": ">=5.0.0",
 	"plugin-type": "theme",
 	"description": "Emphasises individual tiddlers",
-	"dependents": ["$:/themes/tiddlywiki/vanilla"]
+	"dependents": ["$:/themes/tiddlywiki/vanilla"],
+	"plugin-priority": "0"
 }

themes/tiddlywiki/vanilla/plugin.info

@@ -4,5 +4,6 @@
 	"author": "JeremyRuston",
 	"core-version": ">=5.0.0",
 	"plugin-type": "theme",
-	"description": "Basic theme"
+	"description": "Basic theme",
+	"plugin-priority": "0"
 }