SRU request for custom unity-greeter indicators

[Michael Terry]

Hello!  I'd like to request an SRU for unity-greeter that contains a new
feature.  Since this is a feature and not a bug fix, the TB was
mentioned as the appropriate place to request an SRU.

== The bug ==

https://launchpad.net/bugs/1155157

== The rationale ==

A Canonical customer asked for some new features in unity-greeter. 
Specifically:
* The ability to add new indicators to the greeter session
* The ability to change which user is selected in the greeter (done over
DBus)

These changes are only really useful for administrators.  This customer
is using them for automatically selecting a user based on an inserted
smartcard.

Both these features landed in raring.  But for them to be truly useful
to the customer, they would ideally see the light of day in precise.

== The details ==

The patch in that bug adds a new gsettings key
"/com/canonical/unity-greeter/indicators" which lists indicators to
load.  Note that this is pulled from the gsettings database under the
"lightdm" user, not your everyday desktop user.  So a system
administrator would likely add a system gsettings override file to
modify it.  Or call "sudo -u lightdm gsettings".

It also adds a DBus interface with just one function (SetActiveEntry) to
set the currently selected user.  This is exposed only on the session
bus.  Since arbitrary processes cannot be started under the greeter,
there is currently no real consumers for the interface.  But that's
where a custom indicator can come into play.  Such an indicator could
call SetActiveEntry.

== The impact ==

Precise users should theoretically see no changes.  In order to use new
fancy indicators, you'd have to have an administrator specifically add
them.  In order to use the new DBus API, you'd have to have your fancy
new indicator specifically start using it.  No one else should be able
to access the session bus.

Thanks for your consideration,
-mt