Xournalpp: Add capability to change the background pdf

[LittleHuba]

It is currently not possible to change the background PDF later on, without resorting to physically changing the PDF file itself. We should add a method to switch the PDF file used for the background.

This is also in preparation for the new file format, which incorporates the pdf within the xopp file.

[Rmano]

This is quite important, at least for me; I often annotate a PDF file (a solution to a test, for example) only to notice minor errors in the PDF text. Rebuilding it and the be able to reuse the annotation is paramount... Thanks!

[andreasb242]

Even with a new fileformat I would allow to load the file externally, as e.g. LibreOffice also does with Images.

[LittleHuba]

Note for myself: Switch default to attached

[ivanperez-keera]

I know this is not the point of the issue (the point to is to allow it to do from the interface) but, in the meantime, for anyone who is affected by this issue, the xournalpp file is a gz-compressed XML file so you can uncompress it, edit it, and compress it again.

$ mv file.xopp file.gz
$ gunzip file.gz
$ vi file

Find the line with the text:

<background type="pdf" domain="absolute" filename="/some/path/to/some/file.pdf" pageno="1ll"/>

And change the URL to point to the right file, and then:

$ gzip file.gz
$ mv file.gz file.xopp

I think some versions of vim can edit gz files without uncompressing it.

[rolandlo]

You can also remove/rename the old pdf background file. After that upon reloading the xopp-file you will be prompted to specify a pdf-file replacing the old pdf background.