In the console output, trim "latest.xml" from mirror URLs

Usually, this is all you can see, and it's not very helpful.
Download timeout no longer counts time spent queuing or confirming keys

Before, anything that caused a feed import to take longer than 5 seconds
would trigger a call to the mirror site. Now, only delays performing the
download part count.
Replaced "Zero Install" XDG submenu with a single normal menu entry

There's now an Add button in the list window, so that can be used for both, and
you can now also drag-and-drop directly to the list, which seems more natural.

Apart from being a bit cleaner, this should work better on systems which don't
have menus (e.g. GNOME 3, Unity).

Reverts db71bb19958.
Cope with missing selections.xml when running an app

This shouldn't happen, but you might decide to share apps between machines by
checking just the requirements files into version control.

Requested by Tim Cuthbertson.
Improve start-up speed on Arch by storing the path to the distribution package

e.g. the selections document now contains entries like this:

  <selection id="package:arch:gnupg:2.0.19-6:x86_64"
             quick-test-file="/var/lib/pacman/local/gnupg-2.0.19-6/desc"
	     ...>

If the quick test file still exists when we run the app then we know we don't
need to check for updates. Previously, we had to scan /var/lib/pacman/local/
looking for an entry with the right prefix.
Moved gobject initialisation to zeroinstall.support.tasks

This means that importing the main "zeroinstall" module no longer loads gobject
(which is slow). Note that if your program depends on zeroinstall as a library and
uses it to download anything (which requires threads) AND you use gobject/GTK then
you must call gobject.threads_init() before using any gobject code. Previously,
"import zeroinstall" would do this for you.

(currently, "import zeroinstall.support.tasks" will call threads_init, but in
future we may delay initialising it even later)

The immediate benefit from this is that 0install-runenv-runenv.py now runs
faster. On my laptop:

Python 2: 56 ms -> 48 ms
Python 3: 93 ms -> 61 ms
Read ArchLinux desc files binary mode

Avoids UnicodeDecodeError if current locale isn't UTF-8.
Delay initialising gobject until we need to use it

This means that "0install run APP" doesn't need to initialise gobject. On my
laptop, this reduces my test case time from 143 ms to 111 ms (Python 3).
Avoid parsing feeds just to get timestamp

This actually makes no noticable speed difference with a warm cache, but it
might save time after a reboot.
Added support for comment nodes in xmltools.nodes_equal
Fixed support for HTTP chunked encoding with Python 3
Fixed <file> downloads

We forgot to seek to the beginning of the stream before reading it, so the
target file always ended up empty. Unfortunately, the unit-test only checked
that the file existed.
Release 2.2
Start development series 2.2-post
Removed old zeroinstall-add.desktop file