Plot cleanup

Use a helper to make things easier to read.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
First deselect, then select

For reasons I still don't understand, sometimes (but not always) when
clicking on a trip header we appear to get a select notification for the
trip and at the same time deselect notifications for every dive in the
trip. This seems wrong but I can't seem to figure out why it happens -
and of course it causes us to have a mixed up interpretation of what is
selected in our internal selection tracking.

Simply acting on the new selection after the newly deselected items are
handled appears to fix the issue, but I do worry about this change.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Don't offer "delete dive" context menu for trip

If we are not on a dive we shouldn't try to delete it.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>