Document MaxOSX DBUS workaround in the README

This documents the temporary workaround for the libmarblewidget (and
plugin) issues on MacOS in the README.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Much easier way to detect if more than one dive is selected

Instead of getting data from Qt just use our internal notion of the number
of selected dive to decide if more than one dive is selected (and
therefore if we need to show the 'merge dives' option in the context
menu).

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