[e4] Support registration of PreferencePages with E4

[laeubi]

Currently it is not possible to add a PreferencePage using E4 to the usual Window > Preferences because org.eclipse.ui.internal.dialogs.WorkbenchPreferenceManager only handles pages from the extension point org.eclipse.ui.preferencePages even though the PreferenceManager itself is already looked up by the E4 Context.

The goal would be to have a new Type preferencePages under the application model:

here one can add new PreferencePage with the properties:

• class
• label
• category

because of the way E4 works, we don't need any specialization like IWorkbenchPreferencePage and can use the plain JFace PreferencePage here.