Interactive list aircraft display and delete changes

Create two different Time-To-Live parameters for the interactive display
list.

1) Modes.interactive_display_ttl defines for how long the display of an
aircraft persists after the last received message. The default is
MODES_INTERACTIVE_DISPLAY_TTL, which is set to 60 seconds. This replaces
the previous Modes.interactive_ttl/MODES_INTERACTIVE_TTL settings (which
were also 60 seconds)

2) Add an additional Modes.interactive_delete_ttl  parameter, and
default it to  MODES_INTERACTIVE_DELETE_TTL, shich is set to 300
seconds/5 minutes. This defines for how long an aircraft persists in the
list structure after the last received message before being deleted.

Also tidy up a few comments.