[Enhancement] Improve the functionality of the sct_viewer
[Enhancement] Improve the functionality of the sct_viewer

Refactored the code and added subclasses to Clickviewer spicializing
for each command.

Fixes #1205


Former-commit-id: 6d198162ba3fa7bdc38457c3ad2fc0f5f74a1238
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox