FileChooser: Return selected filter

This implements handling the 'current_filter' return
value which returns the filter that was selected in the
file chooser and was added to 'xdg-desktop-portal'
and thus the FileChooser API in commit [1].

Keep a mapping between the user-visible names
(for name filters) or MIME types (for MIME type
filters) to the actual filters in order to retrieve
the actual filter to return after running the file dialog.

Since multiple name filters can use the same filter string,
those aren't necessarily unique, so use the user-visible
names rather than just the filter string to avoid
potential name clashes and be able to retrieve the correct
filter again in the end.

Mapping back to an actual filter does not necessarily
work in all cases, e.g. if a manual filter string was
entered in the file widget. No 'current_filter' is
returned in that case.

This commit moves the extraction of the filters to a
separate method to avoid code duplication.

Handling 'current_filter' is e.g. needed to make gtk's
'gtk_file_chooser_get_filter' function work for the portal
use case. The filter returned by 'gtk_file_chooser_get_filter'
(not just the file extension) is used e.g. by Firefox and
Thunderbird to determine what file format to save a file in,
s. Mozilla bug [2] and gtk issue [3] for more details.

This way, this commit makes that scenario work for the
KF5 native file chooser as well (with 'GTK_USE_PORTAL=1'
set and 'xdg-desktop-portal-kde' in use).

Note: For the case of a gtk program, like the Firefox use case,
this depends on the gtk commits [4] and and [5] in addition
to xdg-desktop-portal commit [1] to actually work as expected.

[1] https://github.com/flatpak/xdg-desktop-portal/commit/35fca7fae881bdaba1bebccf7775eba84407a488
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1517074
[3] https://gitlab.gnome.org/GNOME/gtk/-/issues/1820
[4] https://gitlab.gnome.org/GNOME/gtk/-/commit/baddc90c26f65a0867aa6b4073e1a6b689d87136
[5] https://gitlab.gnome.org/GNOME/gtk/-/commit/d59b28d5f4e2c4ff4cb51df0a39d94c7df7fee96