more url scheme handling

[totaam]

• r3271 added support for URL association, so xpra would be used for opening URLs that look like this: xpra://....
• r3278 added URL handling on win32
• r13106 added support for SSL using xpras://.... (inc win32 changes)
• r12380 equivalent macos bits

See also:

• r9571: xpra session file launch fixes #880: add xpraconfig mimetype
• r3346: desktop-file-validate

We should support more transport types using a more generic scheme:

• xpra:// for TCP
• xpra+ssl:// for SSL
• xpra+ssh:// for SSH
• xpra+ws:// for websockets
• xpra+wss:// for secure websockets

Related: Web-based protocol handlers can be used to register protocols with the browser.

[totaam]

As per rfc2396 appendix A: the scheme should start with a letter and can contain letters, numbers, "+", "-" and "."
But it looks like there are compatibility issues with badly written scheme parsers and the "+" character..

Once this is finished, we should register them here: https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml

[totaam]

Done in r19856 and tested on:

• win32: works fine (used browsers to enter the URL)
• macos: URL mapping works and it fires xpra, but the client does not connect (unrelated problem): macos url handler does not open #1905
• linux: what a load of crap, we added the new URL schemes to the desktop file, and during RPM install we still call:

/usr/bin/update-mime-database
/usr/bin/update-desktop-database

We still validate the desktop file during rpmbuild.
Yet nothing happens, no warnings, no errors, nothing.
But the URL mapping doesn't take:

$ xdg-open xpra+tcp://192.168.1.7:10000/
gio: xpra+tcp://192.168.1.7:10000/: The specified location is not supported

The "legacy" one still does though:

xdg-open xpras://192.168.1.7:10000/

What on earth do we need to do to get the system to update the mapping?
(and for the record, I've tried "xpratcp", "xpra-tcp" and "xpra+tcp" as schemes - no difference, none get registered)
There will not be a "year of the Linux desktop", it's an endless waste of time.

[totaam]

So xdg-open just delegates opening URLs and files to other utilities, depending on what OS and DE is running.

In the case of gnome3 that's "gio" which is part of "glib2".
Turning on all debugging with:

G_DBUS_DEBUG=all xdg-open xpra+tcp://host:port/

We get tons of dbus calls, it seems to query org.gtk.vfs.Daemon.
And also org.gtk.vfs.MountTracker ListMountableInfo.

Other scheme types fail in different ways, equally undecipherable.

[totaam]

Well, that's interesting: this problem doesn't occur on any other test systems, only my main development one. PITA.
No amount of running update-desktop-database or update-mime-database does any good.

r20271 fixes URL scheme patching, this will do.

@maxmylyn: the URL in the ticket description should fire xpra on all platforms (with the correct transport type).
ie:

xdg-open xpra+ssl://192.168.1.6:10000/