When applying removal on customized features' packages, ensure we set meta-package's dependencies as user installed.

This does not permit to apt-get/aptitude to autoremove packages that the user wants in the feature.
2.0.1
Implemented purge support for a list of packages per feature defined in features.py
If an exception occours, show only the error and not the entire traceback in the dialog.
2.0.2
engine: loop through deplist with the auto-hack to get every bit of the feature remvoed.

This slows down things, but the user will be sure that the feature and its dependency tree are fully removed.
2.0.3
Partially revert d955f1a0a8bb0a1a1624d93f5a0229f4188671c0.

Now the autoremoval should work until the user personalizes the feature.
The previous thing, while working correctly, was too slow to be handled in low-end machines.
2.0.4
Added ugly code to show the window ASAP and then calculate the features in the background.