(core) Implement UI for trigger formulas.

Summary:
- Implement UI with "Apply to new records" and "Apply on record changes"
  checkboxes, and options for selecting which changes to recalculate on.
- For consistency, always represent empty RefList as None
- Fix up generated SchemaTypes to remember that values are encoded.

Included test cases for the main planned use cases:
- Auto-filled UUID column
- Data cleaning
- NOW() formula for record's last-updated timestamp.
- Updates that depend on other columns.

Test Plan: Added a browser test.

Reviewers: jarek

Reviewed By: jarek

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D2885

app/common/schema.ts

@@ -31,7 +31,7 @@ export const schema = {
     summarySourceCol    : "Ref:_grist_Tables_column",
     displayCol          : "Ref:_grist_Tables_column",
     visibleCol          : "Ref:_grist_Tables_column",
-    recalcWhen          : "Text",
+    recalcWhen          : "Int",
     recalcDeps          : "RefList:_grist_Tables_column",
   },
 
@@ -204,8 +204,8 @@ export interface SchemaTypes {
     summarySourceCol: number;
     displayCol: number;
     visibleCol: number;
-    recalcWhen: string;
-    recalcDeps: number[];
+    recalcWhen: number;
+    recalcDeps: ['L', ...number[]]|null;
   };
 
   "_grist_Imports": {