Use desktop portal for opening files, if configured

[brosasaki]

Is there an existing issue for this?

• I have searched the existing issues

Problem

Zathura does not seem to use the "desktop portal" for opening files (i.e. the system file selection dialog many programs use when they want to ask the user for a file). I think it would be nice to have, as it would allow users/distros to have their file browsing appear the way they want it to, and it would make the program feel more native and "polished".

Expected behavior

The user presses 'o', or whatever key they have bound, and their system's (and/or their own) customized file picking interface appears, instead of them having to use Zathura's built in interface (that is, unless they have configured it otherwise).