also get rid of some TODOs

noticing broken items, even when they are not touched. Also coloring
their parents in an orange color, so that the error is noticable even
when not expanded.

For better generality (and was useful for error reporting): All widgets
introduced inherit from a common base class: CustomWidget. It holds a
signal definition that can be used to directly forward signals to its
QTreeWidgetItem underneath.

# Conflicts:
#	source/RobotAPI/gui-plugins/SkillManagerPlugin/SkillManagerMonitorWidgetController.cpp