Revert "Keep the dash below the top panel."

Another actor on top is preventinf the drag and drop outside the overview.

This reverts commit 446a3652319bdb38fce74319a277ef95c1c983ed.
Move dash below toppanel only while sliding on messageTray show.

Following previous commit, the dash is now kept below the top panel only while
it's slided up upon messageTray show.