Desklet updating checked in

[Robert kebernet Cooper]

The DeskletUpdater class has been checked in. Desklets now have the
option to update on startup.

You can specify updating in one of 2 ways:

1. Specify a <source> tag with a URL of the authoritative copy of the
desklet. AB5k will check the Last-Modified header on the URL and
compare it to the install directory for the desklet. If newer, it will
uninstall the current version and install the new one.

2. Specify a <source-def> wih the URL to an authortitative copy of the
desklet.xml file AND a <source> tag. AB5k will DL this file, and
compare the <version> tag to the currently installed copy. If
*different* it will download and install from the location in the
<source> tag of the *remote* file. This is for people who might be
moving version around, want guranteed versioning, or are using a DB
and are too lazy to property service an HEAD request :P