(core) Conditional formatting rules

Summary:
Adding conditional formatting rules feature.

Each column can have multiple styling rules which are applied in order
when evaluated to a truthy value.

- The creator panel has a new section: Cell Style
- New user action AddEmptyRule for adding an empty rule
- New columns in _grist_Table_columns and fields

A new color picker will be introduced in a follow-up diff (as it is also
used in choice/choice list/filters).

Design document:
https://grist.quip.com/FVzfAgoO5xOF/Conditional-Formatting-Implementation-Design

Test Plan: new tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: alexmojaki

Differential Revision: https://phab.getgrist.com/D3282

sandbox/grist/docmodel.py

@@ -12,6 +12,7 @@ import six
 import records
 import usertypes
 import relabeling
+import lookup
 import table
 import moment
 from schema import RecalcWhen
@@ -26,6 +27,14 @@ def _record_set(table_id, group_by, sort_by=None):
   return func
 
 
+def _record_ref_list_set(table_id, group_by, sort_by=None):
+  @usertypes.formulaType(usertypes.ReferenceList(table_id))
+  def func(rec, table):
+    lookup_table = table.docmodel.get_table(table_id)
+    return lookup_table.lookupRecords(sort_by=sort_by, **{group_by: lookup._Contains(rec.id)})
+  return func
+
+
 def _record_inverse(table_id, ref_col):
   @usertypes.formulaType(usertypes.Reference(table_id))
   def func(rec, table):
@@ -73,6 +82,8 @@ class MetaTableExtras(object):
     summaryGroupByColumns = _record_set('_grist_Tables_column', 'summarySourceCol')
     usedByCols = _record_set('_grist_Tables_column', 'displayCol')
     usedByFields = _record_set('_grist_Views_section_field', 'displayCol')
+    ruleUsedByCols = _record_ref_list_set('_grist_Tables_column', 'rules')
+    ruleUsedByFields = _record_ref_list_set('_grist_Views_section_field', 'rules')
 
     def tableId(rec, table):
       return rec.parentId.tableId
@@ -83,6 +94,12 @@ class MetaTableExtras(object):
       """
       return len(rec.usedByCols) + len(rec.usedByFields)
 
+    def numRuleColUsers(rec, table):
+      """
+      Returns the number of cols and fields using this col as a rule col
+      """
+      return len(rec.ruleUsedByCols) + len(rec.ruleUsedByFields)
+
     def recalcOnChangesToSelf(rec, table):
       """
       Whether the column is a trigger-formula column that depends on itself, used for
@@ -91,9 +108,10 @@ class MetaTableExtras(object):
       return rec.recalcWhen == RecalcWhen.DEFAULT and rec.id in rec.recalcDeps
 
     def setAutoRemove(rec, table):
-      """Marks the col for removal if it's a display helper col with no more users."""
-      table.docmodel.setAutoRemove(rec,
-        rec.colId.startswith('gristHelper_Display') and rec.numDisplayColUsers == 0)
+      """Marks the col for removal if it's a display/rule helper col with no more users."""
+      as_display = rec.colId.startswith('gristHelper_Display') and rec.numDisplayColUsers == 0
+      as_rule = rec.colId.startswith('gristHelper_Conditional') and rec.numRuleColUsers == 0
+      table.docmodel.setAutoRemove(rec, as_display or as_rule)
 
 
   class _grist_Views(object):