package/atkmm2_28: new package

This is the version needed for gtkmm3.

Note that 2.28 is indeed the latest version before 2.36. They skipped
versions 2.30, 2.32 and 2.34.

Signed-off-by: Daniel Lang <d.lang@abatec.at>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>