(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/test_summary2.py

@@ -927,13 +927,14 @@ class TestSummary2(test_engine.EngineTestCase):
       ]),
     ])
     # We should now have two sections for table 2 (the one with two group-by fields).
-    self.assertTableData('_grist_Views_section', cols="subset", data=[
+    self.assertTableData('_grist_Views_section', cols="subset", rows=lambda r: r.parentId, data=[
       ["id",  "parentId", "tableRef"],
       [1,     1,          4],
       [2,     2,          3],
       [3,     3,          4],
     ])
-    self.assertTableData('_grist_Views_section_field', cols="subset", data=[
+    self.assertTableData(
+      '_grist_Views_section_field', cols="subset", rows=lambda r: r.parentId.parentId, data=[
       ["id", "parentId", "colRef"],
       [1,     1,          24],
       [2,     1,          25],