bt2: InactivityNotification, _Event: use mapping protocol for clock values

bt2: InactivityNotification, _Event: use mapping protocol for clock values

The bt2._Event and bt2.InactivityNotification classes both have the
clock_value() and add_clock_value() methods to get and add clock values
associated to clock classes. This can be replaced with a mapping
protocol accessible with the `clock_value` property.

The mapping's add() method can be used to add a clock value to the
mapping (instead of using __setitem__() because the clock value has a
reference to its clock class (the key) anyway).

Tests are updated accordingly.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>