Reduce amount of api calls.

[Diyou]

As a follow up of #205 :

In a workflow which looked like:

Updater updater("foo");
updater.describeAppImage();
updater.checkForChanges();
updater.start();

updateInformation->buildUrl

Will be called 5 times which will always return the same string I suppose.

[Diyou]

Semantically I think that there should not be any reason for describeAppImage to fetch any data at all to be congruent with an offline context.

[TheAssassin]

Semantically I think that there should not be any reason for describeAppImage to fetch any data at all to be congruent with an offline context.

This method is not supposed to be used in an offline context, though. It never has been. It's also not needed for regular API use.

The API may not be the most efficient (and lazy-initializing/caching data is tricky), it could certainly do better. Please don't hesitate to send pull requests.