Add volume buttons zoom

[Tmarinus]

Added new page into options:

When page turning is on, volume buttons are overwritten to now change the page.
Zoom toggle enables/disables keeping the zoom on the level it was.

I had to implement a channel for the page turning events cause otherwise quick presses would cause bugs.
When using the fixed zoom, there is a very short moment where the full page is show. This is caused by a double call, first setPageIndex followed by zoomTo. When just using zoomTo this issue is significant worse.

        pdfFragment.setPageIndex(currentPageIndex + pageOffset, false)
        if (defaults.isKeepZoom()) {
            pdfFragment.zoomTo(zoomArea, currentPageIndex + pageOffset, 0)
        }

This pull request is related to #296 #271 #290

Please let me know if changes are needed for merging.