Add a 'Clear bulk software queue?' warning / notification before clearing queue

[danboid]

I checked out welcome's software selection for the first time yesterday under 18.04b2 i686. I went through the various categories, picking the apps I wanted but ended up losing my selection in a single click by accidentally clicking on the back arrow (in the top left corner of the welcome software boutique page of welcome), not knowing it would clear my bulk queue of software selections without giving me any prompt first.

The user should get asked if they would like to quit the software boutique and lose their selections when they click on the back arrow instead of it happening silently.