cmake: Additional controls to customize required/optional dependencies

[lgritz]

New cmake variables REQUIRED_DEPS and OPTIONAL_DEPS.

REQUIRED_DEPS is a list of individual package dependencies that are
ordinarily optional (meaning, if not found, the build continues,
possibly disabling some functionality or format support), and forcing
them to be required (build failure if not found). Note that "ALL"
means everying.

OPTIONAL_DEPS is a list of individual package dependencies that
makes them optional even if ordinarily (or via REQUIRED_DEPS) would
be treated as optional. Extreme caution necessary! If you take
something required and force it to be optional, there's no telling
what else in the build might break as a result.

A typical use case might be if you have automated builds and want to be
sure that if for some reason they can't find all the dependencies (even the
optional ones), you will definitely fail the build. But maybe you have some
exceptions. For example,

cmake ... -DREQUIRED_DEPS=ALL -DOPTIONAL_DEPS="DCMTK;R3DSDK"

That means that you want any missing package dependencies to cause the build
to fail, except for missing DCMTK and R3DSDK, which you don't care about.

Also did some cleanup to turn PNG into an optional dependency, including
graceful disabling of ICO if libpng is not found.