Allow uninstalling services/applications

Created by: fonfon

Currently it is only possible to install packages, but not to uninstall them.

There could be a separate page to uninstall software, or there could be an 'Uninstall' button for each app. But I think a 'generic' way that is not dependent on the package itself would be better. First thought is that we move the required packages of each module into __init__.py as required_packages and use this information when uninstalling apps.

This may cause problems if plinth module A and plinth module B both require the same package. When we remove A or B, the other package might get removed also.

One way to solve this problem is to create one debian package per plinth module (e.g. freedombox-tor) and resolve the dependencies there, but that would create a lot of packaging work.