Recording average depth data

This is the calculation that is needed to display a running average in the
profile.

It adds a new member plot_data.running_sum which can be turned into the
running average by dividing by plot_data.time.

Right now this isn't used by the UI.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix compilation with upstream Marble

<marble/MarbleDebug.h> is not availaible in all Marble versions.
As Marble::MarbleDebug::setEnabled(verbose) is only used when
MARBLE_SUBSURFACE_BRANCH is defined, also include header file only when
MARBLE_SUBSURFACE_BRANCH is set.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>