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/actions/actionDeleteSelected.tsx

@@ -18,13 +18,14 @@ export const actionDeleteSelected: Action = {
   contextMenuOrder: 3,
   commitToHistory: (_, elements) => isSomeElementSelected(elements),
   keyTest: event => event.key === KEYS.BACKSPACE || event.key === KEYS.DELETE,
-  PanelComponent: ({ updateData }) => (
+  PanelComponent: ({ elements, updateData }) => (
     <ToolButton
       type="button"
       icon={trash}
       title={t("labels.delete")}
       aria-label={t("labels.delete")}
       onClick={() => updateData(null)}
+      visible={isSomeElementSelected(elements)}
     />
   ),
 };