More mobile tweaks (#790)

* Disable text selection

* Set content-editable=plaintext-only to disable Touch Bar formatting buttons

* Enlarge resize handle tap targets for pen/touch

* Make the lock button a button in mobile mode

* Use icons instead of Unicode characters; add an alternate toolbar for creating multipoint lines

* Allow buttons to hide themselves

* Fix heuristic for showing shape actions

* Refactor icons

* Fix label for edit button

* Switch edit button icon

* Remove lock button on mobile

* Add language selector on mobile

* Fix showing edit button on mobile

* Fix showing edit button on mobile, part 2

* Fix handle touch regions

* Fix scroll-back button position

* Allow using the text tool on a text object to start editing it

* Fix deletion of last point in line

src/components/ToolButton.tsx

@@ -15,6 +15,7 @@ type ToolButtonBaseProps = {
   size?: ToolIconSize;
   keyBindingLabel?: string;
   showAriaLabel?: boolean;
+  visible?: boolean;
 };
 
 type ToolButtonProps =
@@ -45,6 +46,10 @@ export const ToolButton = React.forwardRef(function(
         type="button"
         onClick={props.onClick}
         ref={innerRef}
+        style={{
+          visibility:
+            props.visible || props.visible == null ? "visible" : "hidden",
+        }}
       >
         <div className="ToolIcon__icon" aria-hidden="true">
           {props.icon || props.label}