Reset per-package variants if corresponding package is uninstalled

[memo33]

With the new per-package variant names group:pkg:variant, we should remove these package-local variants when the corresponding package is uninstalled.

In the unlikely case that another package makes use of the same variant, a new selection dialog would be triggered at the next update. (Usually, packages using foreign package-local variants should depend on the original package, which would avoid this.)

Additionally, if a variant is prefixed with a package name, the variantDescriptions should be taken from that package as a fallback. Currently, these are only taken from the package that makes use of the variants.

Moreover, there should be a way to add a text field to the variantDescriptions that is displayed in the selection dialog before all the variant names.