Add progress indicator when the storage panel is loading

Sam Parkinson requested to merge sam/gnome-usage:storage-progress into master

This commit changes the spinner into a progress bar. It is updated with information from the storage analyzer.

The storage analyzer was changed to add another message type, updating the size. This is emitted as quickly as possible, so that the user gets the most accurate and optimistic view of the progress.

Here is a picture of the progress bar:


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