Person: Deleting persons - Fixes issue #19

Delete persons by right clicking and selecting "Delete".

Make "Properties" menu item also pop up on right clicking.

Only one Properties box should be visible at a time, i.e. if one
Person's properties box is showing up, selecting another Person's
property box makes the first one go invisible.

Don't make creation of persons dependent on the creation of events.
Timelines: Delete timelines - Fixes issue #19

Right click on a timeline, select "Delete"

Issues: If you delete a timeline created earlier, there will be an area
of blank space between the timeline placed before the one deleted and
the timeline placed after the one deleted.
RESET this