(core) Create an extra raw data widget when creating a table

Summary: This is the first step towards raw data views, merely adding metadata without any UI. Every 'normal' table now has a widget referenced by `rawViewSectionRef`. It has no parent view/page and cannot actually be viewed for now. The widget is created during the AddTable user action, and the migration creates a widget for existing tables.

Test Plan: Many tests had to be updated, especially tests that listed all view sections and/or fields.

Reviewers: jarek, dsagal

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3232

sandbox/grist/schema.py

@@ -15,7 +15,7 @@ import six
 
 import actions
 
-SCHEMA_VERSION = 25
+SCHEMA_VERSION = 26
 
 def make_column(col_id, col_type, formula='', isFormula=False):
   return {
@@ -55,7 +55,9 @@ def schema_create_actions():
 
       # A table may be marked as "onDemand", which will keep its data out of the data engine, and
       # only available to the frontend when requested.
-      make_column("onDemand",     "Bool")
+      make_column("onDemand",     "Bool"),
+
+      make_column("rawViewSectionRef", "Ref:_grist_Views_section"),
     ]),
 
     # All columns in all user tables.