Use signals and decorators for configs again.

This basically rolls back 64a119afb26eaf37f95d3748af75ff35678a54e7 and
b7f2a6d1435810434282ce8a6887fc744e08dbfe.

Fixes #188.
Breaks #156 again but the next commit will fix this.
Don't update stylesheets of deleted objects.

Fixes #156.