(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

test/nbrowser/gristUtils.ts

@@ -1479,6 +1479,14 @@ export async function addColumn(name: string) {
   await waitForServer();
 }
 
+// Select a range of columns, clicking on col1 and dragging to col2.
+export async function selectColumnRange(col1: string, col2: string) {
+  await getColumnHeader({col: col1}).mouseMove();
+  await driver.mouseDown();
+  await getColumnHeader({col: col2}).mouseMove();
+  await driver.mouseUp();
+}
+
 /**
  * Changes browser window dimension to FullHd for a test suit.
  */