Add SchemeSupported to check whenever host supports opening specific uri scheme

[xhorak]

Operating System

Linux

XDG Desktop Portal version

Git

XDG Desktop Portal version (Other)

No response

Desktop Environment

GNOME

Desktop Environment (Other)

No response

Expected Behavior

As a workaround currently Firefox has a list of the supported URI schemes but there are many and number is growing, so it's not covering all the cases obviously.

I'd like to implement SchemeSupported method to the portal.AppChooser which returns true or false whenever the system has handler application for the specific uri scheme or not.

xdg-desktop-portal-gtk issue: flatpak/xdg-desktop-portal-gtk#330
Upstream bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1786132

Current Behavior

Currently there's no way to decide whenever specific uri scheme is supported in the host system. This confuses browsers which checks system for the uri scheme handler to open the app or open the uri internally.

Steps to Reproduce

...

Anything else we should know?

No response

[xhorak]

Relevant pull request: #1203