Keep focus in overlay editor

[BrianHung]

Tries to fix #910

• if an editor is a native html input or textarea, we use readOnly instead of disabled; this allows the element to be focus-able but not editable. Benefit of this is that selection still works if a person wants to copy the cell content.
• for editors without a focusable element (and also editors in general), instead of using an invisible textarea with autofocus=true, we make the overlay editor focusable itself and autofocused on mount; shouldn't conflict with editors with autofocused elements since children renders after parent

We should add some tests to check that click within cell and then pressing escape works for each internal cell editor.