[RAPPS] may expose the "%ls" to the user unintentionally

0.4.8-dev-11-g07c7295
The issue initially was introduced when merging rapps Gsoc2017 into trunk.
A very tiny GUI glitch only and only added as a reminder for sanchaez:

When download-progress-dlg is visible, the name of the downloaded app is passed into the titlebar with a "%ls". This "%ls" is sometimes exposed to the user unintentionally:
-for a fraction of a second in case of beginning downloading until the app-name is retrieved
-until user confirms message-box of a download that failed (e.g: due to dead URL)