(core) Allow filtering hidden columns

Summary:
Existing filters are now moved out of fields
and into a new metadata table for filters, and the
client is updated to retrieve/update/save filters from
the new table. This enables storing of filters for
columns that don't have fields (notably, hidden columns).

Test Plan: Browser and server tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3138

app/common/schema.ts

@@ -4,7 +4,7 @@ import { GristObjCode } from "app/plugin/GristData";
 
 // tslint:disable:object-literal-key-quotes
 
-export const SCHEMA_VERSION = 24;
+export const SCHEMA_VERSION = 25;
 
 export const schema = {
 
@@ -185,6 +185,12 @@ export const schema = {
     child               : "Ref:_grist_ACLPrincipals",
   },
 
+  "_grist_Filters": {
+    viewSectionRef      : "Ref:_grist_Views_section",
+    colRef              : "Ref:_grist_Tables_column",
+    filter              : "Text",
+  },
+
 };
 
 export interface SchemaTypes {
@@ -366,4 +372,10 @@ export interface SchemaTypes {
     child: number;
   };
 
+  "_grist_Filters": {
+    viewSectionRef: number;
+    colRef: number;
+    filter: string;
+  };
+
 }