Allow opening a specific line / page in an external program

[luleyleo]

I'm not sure, how to approach this with Flatpak and what the UI should look like.

[luleyleo]

https://gitlab.gnome.org/GNOME/Incubator/papers/-/blob/main/shell/src/application.rs?ref_type=heads#L135

For PDFs (Gnome Papers and KDE Okular) the page can be passed as part of the file URI:

file:///path/to/file.pdf#page=x

For text files (KDE Kate, not Gnome TextEditor) this works:

file:///path/to/file.rs?line=40

[luleyleo]

The OpenURI Portal apparently does not support file URIs for some reason. This will probably only be possible with a host service which directly launches the applications with appropriate arguments.