setting aside SAMPLE_EVENT_xxx ranges for applications

Hey Jef,

chasing a bug in Subsurface today I realized that we are abusing existing event types internally since we don't have reserved event types that we know will not be used by libdivecomputer in the future.

I guess there would be two solutions:

a) we ask you to add events that you haven't seen in any dive computer, yet, so we can use them as synthetic events for our own purposes (so far this is only a "setpoint change" event that we use in the planner).

b) you add a reserved range of numbers that we can simply redefine

SAMPLE_EVENT_APP_RESERVED_1 ... SAMPLE_EVENT_APP_RESERVED_8

(or some reasonable number) Then we can do a #define SAMPLE_EVENT_SUBSURFACE_SP_CHANGE SAMPLE_EVENT_APP_RESERVED_1 and all is well.

What do you think?

/D