add white space around pdf

[tubbadu]

it would be very useful a way to add some white space around a pdf in order to take notes beside it, like OneNote does

[bhennion]

Adding a margin on the right hand side of the page and/or at the bottom is doable without altering the file format (if the page width is bigger than the pdf's width, then white space is added, already in the current version of Xournal++).
For more complicated situations, then this will have to be planned into the new file format.

[ngc92]

This seems like a simpler (easier to implement?) version of #1174,
and possibly a duplicate of #1698