(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/components/ViewLayout.css

@@ -3,6 +3,10 @@
   flex: 1 1 0px;
 }
 
+.viewsection_buttons {
+  margin-left: 4px;
+}
+
 .viewsection_title {
   flex-shrink: 0;
   align-items: baseline;