(core) Add cell selection summary

Summary:
Adds a cell selection summary to grid view that shows either a count or
sum of all the selected values. Implementation was done by Dmitry.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: paulfitz, dsagal, jarek

Differential Revision: https://phab.getgrist.com/D3630

app/client/components/GristDoc.ts

@@ -1262,7 +1262,7 @@ const cssViewContentPane = styled('div', `
   flex: auto;
   display: flex;
   flex-direction: column;
-  overflow: hidden;
+  overflow: visible;
   position: relative;
   min-width: 240px;
   margin: 12px;