ToolButton: Fix text not visible

iconSource is an url so it needs to be converted to
a string when used with the non-coerning test checking
if it is an empty string or not. Otherwise it is always false
and the text item never visible.

Add ToolButton to the manual test tool TestBench.

Change-Id: I1099bb4c7c10565e3f40588c82015ae54be42e20
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>