Add apps to the Python package

[lassoan]

Instead of calling the current apps "examples", it would be nicer to call them "apps" and include them in the Python package. It would make it easier for people to get started (they would not require cloning the repository, etc.). The instructions could be something like this:

pip install trame-slicer[standalone]
trame-slicer-viewer /path/to/image.nrrd

or

pip install trame-slicer[standalone]
trame-slicer-segment-editor /path/to/image.nrrd /path/to/segmentation.seg.nrrd

Napari getting started instructions may give some additional ideas: https://napari.org/stable/getting_started/installation.html