(core) Add support for editing on mobile.

Summary:
- Add custom handling for dblclick on mobile, to allow focusing editor.
- In place of Clipboard.js, use a FocusLayer with document.body as the default focus element.
- Set maximum-scale on iOS viewport to prevent auto-zoom.
- Reposition the editor on window resize when editing a cell, which is a normal
  occurrence on Android when virtual keyboard is shown.
- Add Save/Cancel icon-buttons next to cell editor on mobile.

Test Plan: Tested manually on Safari / FF on iPhone, and on Chrome on Android emulator.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2721

app/client/lib/browserInfo.ts

@@ -11,3 +11,10 @@ export function isDesktop() {
   const platformType = getParser().getPlatformType();
   return (!platformType || platformType === 'desktop');
 }
+
+// Returns whether the browser is on mobile iOS.
+// This is used in particular in viewport.ts to set maximum-scale=1 (to prevent iOS auto-zoom when
+// an input is focused, without preventing manual pinch-to-zoom).
+export function isIOS() {
+  return navigator.platform && /iPad|iPhone|iPod/.test(navigator.platform);
+}