Visually mark a form checkbox when it's the focused element

 * Xreader version 2.8.3
 * Distribution Linux Mint 20.1 Ulyssa

Issue

It's not possible to see when a check-box is the focused element while editing a PDF form.

Steps to reproduce

1. Open a PDF form with a check-box, e.g. IRS 1040
2. Click on a check-box to give it the focus.
3. Press space once or twice to see what it looks like both checked and un-checked.
4. Press tab to move the focus to a different form element.

Expected behaviour

The visual appearance of a check-box should be different when it has the focus than when it doesn't, so pressing the tab key above should change how the focused box looks in some way. Some viewers surround it with a dashed line, others sink it like a text control.

Other information

The buggy keyboard navigation of the check-box widget in version 2.4.4 was fixed last year (thanks @icarter09, my build of your branch wasn't working last September but I've upgraded Mint since and the newer version shows that you did fix it) but this task was left to do:

I'll probably make a feature request ticket for that and address via the new ticket.

Thanks in advance!

Reproduce the issue in Xreader 2.8.3 with the linked IRS 1040 PDF: focus a checkbox, toggle it with Space, and move focus with Tab. Trace the PDF form checkbox widget handling and verify that checked and unchecked focused boxes have a distinct visual appearance from unfocused boxes.