[RAPPS] aesthetics issue: download progress sticks with previous download

0.4.9-RC-27-g39f5e56 & 0.4.10-dev-126-gea42633

This is a trivial aesthetics issue only.

Reproduction steps
-download an arbitrary app via rapps (and wait for the download/installation to finish)
-download another arbitrary app via rapps
-watch the dialog that contains the progress bar closely before the second download starts (this may take up to several seconds depending on your connection speed)

Observed behavior
-the progress dialog shows the download size of the first (already completed download) erroneously

Expected behavior
-the progress dialog should show either the size of the second download (or nothing at least) but not the size still of the first download

Suggestion:
We should most likely clear some global variables here that contain the download size either right when we successfully finished a download, or when we start a new one.