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
