ui: dynamic page allocation to gtk grid

[jchilds0]

Implements #719 to resolve in #449, #365, #253, #200, and #65

Tested on the ARM reference manual (15,000 pages) https://developer.arm.com/documentation/ddi0487/latest, and Intel manual (5000 pages) https://developer.arm.com/documentation/ddi0487/latest, and doesn't crash when zooming in to either document.

I'm not sure if the document widget should stay as a GTK widgt or change to a normal struct, I'm fine with either.

Care needs to be taken when manually setting the value of the ui viewport, since the document widget changes the grid. zathura_document_widget_[set_ratio_from_value|get_ratio] functions wrap zathura_adjustment_[set_ratio_from_value|get_ratio] functions to handle the conversion from document ratio to widget ratio.

The canvas size chosen is 32,767 which is the max cairo canvas height (perhaps this can be increased but I don't know how much cairo can handle). zathura_document_widget_render updates the pages in the grid when the current page is within 1 page of the current pages shown. There's probably a smarter way to handle this as it doesn't do to well with scrolling in zoomed out documents.