Bug 500061 - Compare editor does not link with Project Explorer

Add an ILinkHelper to link the EditorInput of a CompareEditor to the
CommonNavigator.
- The LinkHelper links the IFile of the active sub-editor to the
CommonNavigator, when the CompareEditor part is activated.
- The link is *not* refreshed when the active editor in the
CompareEditor is changed, either due to user input, or the editor input
becoming available. The ILinkHelper is only consulted on
'partActivated'.

Change-Id: I2bbb4b9aac2826a8880e626782369a49228d88ad
Signed-off-by: Rolf Theunissen <rolf.theunissen@altran.com>
4 files changed