(core) Context menu for cards.

Summary:
User was not able to delete cards. This patch introduces a context menu for cards, analogous to the one available for rows on a GridView.

Changes:
- Row numbers on a GridView have the same icon as on columns to make context menu more discoverable.
- Context menu for rows and columns, when activated, didn't switch section in rare conditions (i.e. when the section had 2 or more columns selected, one of which had the same rowId as a column in the section that the user switched from).
- Card list layout and a single card layout has the same context menu as in a GridView, available by pressing the context menu button.

Test Plan: Browser tests

Reviewers: dsagal, paulfitz

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2870

app/client/ui/MenuToggle.ts

@@ -0,0 +1,34 @@
+import { dom, DomArg, IDisposableOwner, styled } from "grainjs";
+import { icon } from "app/client/ui2018/icons";
+import { colors } from "app/client/ui2018/cssVars";
+
+/**
+ * Creates a toggle button - little square button with a dropdown icon inside, used
+ * by a context menu for a row inside a grid, a card inside a cardlist and column name.
+ */
+export function menuToggle(obs: IDisposableOwner, ...args: DomArg[]) {
+  const contextMenu = cssMenuToggle(
+    icon('Dropdown', dom.cls('menu_toggle_icon')),
+    ...args
+  );
+  return contextMenu;
+}
+
+const cssMenuToggle = styled('div.menu_toggle', `
+  background: white;
+  cursor: pointer;
+  --icon-color: ${colors.slate};
+  border: 1px solid ${colors.slate};
+  border-radius: 4px;
+  &:hover  {
+    --icon-color: ${colors.darkGreen};
+    border-color: ${colors.darkGreen};
+  }
+  &:active  {
+    --icon-color: ${colors.darkerGreen};
+    border-color: ${colors.darkerGreen};
+  }
+  & > .menu_toggle_icon {
+    display: block; /* don't create a line */
+  }
+`);