Re-enable Right-Click on DhListView columns [LT-14784]

Some of the code for handling Rightclicking was lost in the last
work done on this class. I've added back in the missing code and I
belive it will work on both platforms. Linux will need testing.

Change-Id: Ifcb1a348330a9f9e37cd4c523d46f8ccd82754b5