(core) Freezing columns on a GridView

Summary:
User can freeze any number of columns, which will not move when a user scrolls grid horizontally.
Main use cases:
- Frozen columns don't move when a user scrolls horizontally
- The number of frozen columns is automatically persisted
- Readonly viewers see frozen columns and can modify them - but the change is not persisted
- On a small screen - frozen columns still moves to the left when scrolled, to reveal at least one column
- There is a single menu option - Toggle freeze - which offers the best action considering selected columns
- When a user clicks a single column - action to freeze/unfreeze is always there
- When a user clicks multiple columns - action is offered only where it makes sens (columns are near the frozen border)

Test Plan: Browser tests

Reviewers: dsagal, paulfitz

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2852

app/client/ui2018/cssVars.ts

@@ -43,6 +43,8 @@ export const colors = {
 
   cursor: new CustomProp('color-cursor', '#16B378'),   // cursor is lightGreen
   selection: new CustomProp('color-selection', 'rgba(22,179,120,0.15)'),
+  selectionOpaque: new CustomProp('color-selection-opaque', '#DCF4EB'),
+
   inactiveCursor: new CustomProp('color-inactive-cursor', '#A2E1C9'),
 
   hover: new CustomProp('color-hover', '#bfbfbf'),