Add ability to issue navigator to find all issues linked to x issue - with option to constrain by link type

A customer was asking for the ability to see all issues linked to an individual issue. This would a nice addition to the issue navigator.

They also wanted the ability to see only linked issues of a certain type so we need the ability to specify the link type (a multi-select list with 'All' and the specific options). The issue navigator can then be customised to display the relevant information / view of those linked issues.

Being able to view the relationships in a graphical format would also be very desirable.