Add a postinst script that safely transitions to the new package formats

[natanjunges]

Expected behavior

The update to a next version should be as uneventful as possible, with previously installed packages correctly detected and set up.

Actual behavior

Since many packages were changed since last release (the global ASCII keys were replaced by local binary keys, with PPAs greatly affected by this, appimagelauncher no longer uses an apt repository), if deb-get is updated without properly handling those changes, many previously installed packages will leave behind potentially harmful stale files.

Proposed solution

A postinst script could be added to detect the previously installed packages, filter which ones are affected by the breaking changes and clean up any files that they left behind.