.desktop file: supported URL schemes key

[David Faure]

The recent discussion raises again something that has been a need for a very 
long time.

Some programs support URLs (Exec=foo %u) but not all URLs.
For instance VLC supports http, ftp, and smb.
We need to know that we can pass it such URLs, and not others.

I would like to propose a key UrlSchemes in the desktop entry standard,
as a ;-separated list of supported URL schemes. For instance, for vlc:
UrlSchemes=http;ftp;smb;

For applications that use a VFS framework, listing all supported URL schemes 
is impossible. Instead, I suggest UrlSchemes=KIO or UrlSchemes=GIO.
This of course raises another issue, which is that KDE code can easily know 
what KIO expands to, but not what GIO expands to. And of course some schemes 
are not necessarily named the same. I don't propose to solve this completely 
now, we can just assume a basic set of schemes for now, but I would already be 
quite happy with a standard UrlSchemes key, for all the solvable cases.

For backwards compatibility, what I did in KDE was to also have logic like:
* If no %u/%U, then no URLs supported.
* If UrlSchemes key is present, use that.
* Otherwise look at Categories, and if it contains KDE, then assume 
UrlSchemes=KIO
* Otherwise fallback to: http;https;ftp; which seems like a reasonable default 
for apps that claim to support URLs.

(In KDE the key is currently named X-KDE-Protocols, but I guess UriScheme is 
the proper name for what the historic kde code has called protocols)

(And the reason for all this is that if the app doesn't support the URL 
scheme, we instead download to a local file, pass to the app, and delete the 
file when the app is closed; so just blindly passing all URLs to the app and 
hoping for the best, isn't really a valid solution)

Does this change to the spec seem reasonable? If yes I'll try to figure out the 
git repo and whether I have write access to it ;)

-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Sponsored by Nokia to work on KDE, incl. Konqueror (http://www.konqueror.org).