Since commit f5c69a5195 the examples/widgets/painting examples could
not be built standalone anymore.
Move the find_package calls out of the examples/widgets/painting/shared
subdirectory and add a .cmake file that can be included to use the
library conveniently.
Change-Id: I35cb5bd8419a67c782d109104c4a88d64477a8be
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>