NEWT EDTUtil: Exposed weakness of EDTUtil usage due to usage of WeakReference, i.e. higher retention of Display instances.

- WeakReference Change 99479bf3197cde8e89c5b499d135417863d521c7

- Refines commits:
  feb352145af1643a57eaae99c0342e6f5e0f2a2e
  dec4b02fe4b93028c85de6a56b6af79601042d6e
  433e3914324b90c910b018bb7d9d80e814c67123

Reviews EDTUtil API and usage:
  - less confusing / more determined EDTUtil API
  - EDTUtil's thread shall only be reset and started when required (-> lazy)
  - EDTUtil's instance in Display shall be handled thread safe w/o extra blocking
  - EDTUtil's implementations (Default, SWT and AWT) shall be aligned / similar as much as possible

Further note: SWT's EDTUtil (NewtCanvasSWT) shall not use a reused Display instance due to it's
              custom SWTEDTUtil. We may need to disable the ref. cache if custom EDTUtil (setEDTUtil)
              is intended (used).