- Rename PreferenceSpyAddon.initialzePreferenceSpy typo to initializePreferenceSpy.
- Rename DeletePreferenceEntries / DeleteAllPreferenceEntries in PreferenceSpyPart
  to camelCase per Java convention.
- PreferenceSpyConfiguration: make BUNDLE_ID a static final constant and drop the
  stray double semicolon.
- Use pattern matching for instanceof in PreferenceSpyPart, CollapseAllHandler and
  ExpandAllHandler.
- Type PreferenceNodeEntry.preferenceEntries as IObservableSet<PreferenceEntry>
  instead of IObservableSet<Object>, simplifying findPreferenceEntry and the
  content provider walker.
- Remove the second setInput call in preferenceChanged: the WritableSet already
  propagates the addition to the viewer.
- Drop the unused setPreferenceEntries setter and removeRecentPreferenceNodeEntry.
- Extract the '*default*' marker used by ShowAllPreferencesHandler into a constant.

Preparation for #2344