"No scroll" widgets refactor

[GriffinRichards]

• Separates from NoScrollComboBox a new parent class ComboBox (which contains all our generic QComboBox improvements) and a new child class EventComboBox (which contains some optimizations for the Events tab that we had to keep working around elsewhere).
• Generalizes the "no scroll" behavior into a single event filter, so we no longer need to keep reimplementing wheelEvent every time we have a widget that doesn't steal focus when scrolling. Only NoScrollComboBox has any special considerations, the rest (like NoScrollAbstractSpinBox) are only provided for convenience / direct usage in forms.
• Fixes a potential pitfall where toggling the editability of a NoScrollComboBox could break the editingFinished signal and the behavior of the completer.