CancelUpdate should emit a Cancelled signal

[John Lenton]

to implement a corner case of the system updates notifications spec (wiki.ubuntu.com/SoftwareUpdates#Prompting) we need to emit a signal from CancelUpdate. Thus, Cancelled.

The use case is this: when a system update notification comes in, if a user has selected automatic download of updates, no notification is presented to the user and instead the download is initiated. Once the download finishes, then the notification is presented.

If, while the first download is being waited for, a second notification comes in, the first download should be canceled, and the process waiting for that download terminated; not doing so could lead to a large number of proccesses hanging around waiting for completion, and a large number of notifications presented in a thundering herd once it completes.

[Barry Warsaw (barry)]

From IRC: We will still continue to emit UpdateFailed if a download is in progress, but Cancelled will always be emitted.

[Barry Warsaw (barry)]

When both signals are emitted, order is unimportant. TBD: specify and guarantee an order or leave it unspecified?

[Barry Warsaw (barry)]

After further discussion, it is agreed that this new API is not necessary to support the UX we want.