TEIIDDES-1831: Provide a status icon in the server status panel

* Adds a server status icon to the status panel in the Model Explorer view

* Indicates whether the default server instance is connected or not

* Image instances should be disposed by the SWT labels

* All calls to the setter methods for the status panel wrapped within a UI
  thread runnable

* Server status panel de-cluttered with the edit link reduced to an icon
  with tooltip.

* Since status label is not initially populated, it is necessary to relayout
  its parent composite after setting its image. Otherwise, the image is
  never displayed.

* UiPlugin
 * Clean up some old commented-out code
 * Use the registry for descriptor rather than individual references
Updates target platform in line with integration stack

* Release of integration stack target version of 4.1.4.Final