also search in optional package

Kittens assumes to have the icons in a icons/<appname> subdirectory of the python module index, but in the new installation layout it can be <pythonpackage>/icons/<appname>. This pull request adds a optional package name to load_icons() where you can define the package name. This is required for fixing this issue:

https://github.com/ska-sa/meqtrees-timba/issues/3