(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/components/viewCommon.css

@@ -1,3 +1,6 @@
+/*
+  record class is used for grid view header and rows
+ */
 .record {
   display: -webkit-flex;
   display: flex;
@@ -10,6 +13,13 @@
   border-color: var(--grist-color-dark-grey);
   border-left-style: solid;  /* left border, against rownumbers div, always on */
   border-bottom-width: 1px; /* style: none, set by record-hlines*/
+  /* Record background is white by default.
+     It gets overridden by the add row, zebra stripes.
+     It also gets overridden by selecting rows - but in that case background comes from
+     selected fields - this still remains white.
+     TODO: consider making this color the single source
+  */
+  background: white;
 }
 
 .record.record-hlines {  /* Overwrites style, width set on element */