maintain #anchors in file:// uris

[pixelb]

This is coming from ptyxis issue 516

I.e., almost all terminals will correctly propagate the #anchor in file:///foo/bar.html#anchor.
Unfortunately ptyxis drops it due to libportal use.

The use case I have is I'm going to markup --help from all of coreutils to link to the appropriate part of a local manual
(a remote manual is fine, but a local manual should work (as it does with most other terminals))

[pixelb]

Note the suggestion discussed in the ptyxis issue was to not treat .html files, or at least .html with #anchors as normal files, and just treat like urls. I.e. be more selective at:

libportal/libportal/openuri.c

Line 200 in 086cc57

if (g_file_is_native (file))