(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/components/Clipboard.js

@@ -62,7 +62,7 @@ function Clipboard(app) {
 
   FocusLayer.create(this, {
     defaultFocusElem: this.copypasteField,
-    allowFocus: isCopyPasteTarget,
+    allowFocus: allowFocus,
     onDefaultFocus: () => {
       this.copypasteField.value = ' ';
       this.copypasteField.select();
@@ -184,10 +184,12 @@ var FOCUS_TARGET_TAGS = {
  * copy-paste events. Besides inputs and textareas, any element can be marked to be a valid
  * copy-paste target by adding 'clipboard_focus' class to it.
  */
-function isCopyPasteTarget(elem) {
+function allowFocus(elem) {
   return elem && (FOCUS_TARGET_TAGS.hasOwnProperty(elem.tagName) ||
     elem.hasAttribute("tabindex") ||
     elem.classList.contains('clipboard_focus'));
 }
 
+Clipboard.allowFocus = allowFocus;
+
 module.exports = Clipboard;