Button instead of ALT key to switch between text and area annotation on iOS

[cweniger]

Text highlighting works on iOS flawlessly as far as I can see (using the Pen), which is fantastic since I can read PDFs and annotate them on the tablet. Area annotation does not work since the ALT key is missing, however. One solution might be to add a button that allows switching between area and text annotation. That seems to be related to this pull request for react-pdf-highlighter

agentcooper/react-pdf-highlighter#106