[zathura] [wish] fit window to page

[Maxim Gonchar]

I lack the ability to fit the window to the page dimensions and keeping the window fit to the page when zooming. Like
mupdf does when you press 'w'.

Also it's a bit strange, that when you use zathura with statusbar and fit a page to window, the lower part of the page
becomes covered by the statusbar. If you manually resize the window so the statusbar doesn't cover the bottom of the
page, you can see the bottom of the previous page on top, or the top of the next page on bottom for the first page.

Thank you for working on zathura!

regards,
Maxim