(core) Implement 'Print widget' option to print individual view sections.

Summary:
- Supports multi-page printing with some aggressive css overrides.
- Relies on a new function implemented by grist-plugin-api to print a
  multi-page CustomView.
- Renders all rows for printing for scrolly-based views.

Test Plan:
Doesn't seem possible to do a selenium test for printing. Tested
manually on Chrome, Firefox, and Safari.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2635

app/client/components/GridView.css

@@ -71,6 +71,7 @@
   left: 0px;
   overflow: hidden;
   width: 4rem; /* Also should match width for .gridview_header_corner, and the overlay elements */
+  flex: none;
 
   border-bottom: 1px solid var(--grist-color-dark-grey);
   background-color: var(--grist-color-light-grey);
@@ -89,6 +90,15 @@
   .gridview_data_row_num {
     background-color: var(--grist-color-light-grey) !important;
   }
+  .gridview_header_backdrop_top {
+    display: none;
+  }
+  .column_name.mod-add-column {
+    display: none;
+  }
+  .gridview_data_header {
+    background-color: var(--grist-color-light-grey) !important;
+  }
 }
 
 /* ========= Overlay styles ========== */