Redesign of the legacy app

[kirillt]

This older implementation should be reworked with better drawing capabilities and ARK support:
https://github.com/ARK-Builders/ARK-PenAndPDF-Legacy

A drawing library could be extracted from ARK Retouch and used for drawing on pages.

For the start, this should be implemented:

1. The app can invoke ark-filepicker and select a PDF file using it. ark-filepicker should be used due to its "roots" tab, which simplifies access to folders registered as ARK mounting points.
2. The PDF selected can be viewed, user can scroll pages by dragging motion over screen borders. Bottom scrollbar also could be displayed allowing quicker selection of necessary page. User should be capable of hiding the bottom scrollbar by tapping on any point of the document.
3. The app must have a floating [edit] button. When it's tapped, user gains capability to edit current page. All applicable operations from ARK Retouch should be supported by extracting common code into a library. Cropping or rotation are not needed for this app. Drawing with any color, thickness and transparency level should be accessible with aid of new buttons. All control buttons should be hided by tap though. Scrolling pages should be disabled in drawing mode. Tapping [edit] button again must switch mode back to viewing. If there are any unsaved changes, another floating button [save] must appear. For now, saving the document could be just overwriting the original file. Saving into separate document will be implemented later together with versions tracking.