Debugger: Fix unpinned tooltip expansion

True expansion was setting another WidgetContent to
an exististing Tooltip. Arguably wrong in the base
Tooltip handling, but avoidable on the debugger side,
too. This was visible only on Mac.

Change-Id: I2e5a3273b571658b4dd4200c9b3a0e9542a16015
Reviewed-by: hjk <hjk121@nokiamail.com>