Add selection coordinates to command exec parameters

Open pcy requested to merge pcy/zathura:selection-exec-params into develop

This series of two commits adds the parameters $X1, $Y2, etc. to the parameters available to a shell command.

This is quite useful as this allows one to create a keybinding that will cryptographically sign the document using several types of government-issued ID cards using e.g. pyHanko. These types of signatures need to be placed at certain coordinates on the PDF document (as opposed to eg. PGP signatures), and are the only type of signatures governments etc. will typically acknowledge (also unlike PGP signatures). An example of this usage can be found here.

The first commit adds two API functions that expose the selection rectangle coordinates to a page, so I could use those in the second commit. I'm not sure if this is the best place to add these two functions or if there's a better way to do this, input is welcome there.

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