core: create qPrefPartialPressureGas from SettingsObjectWrapper

Update set/get functions to follow common name scheme:
- get function have same name as in struct diveComputer
- set function have set_<name>
- signal function have <name>_changed

one class one .h/.cpp is the C++ idiom. Having load/sync of each
variable in 1 functions (in contrast to the distributed way
SettingsObjectWrapper handles it) secures the same storage name
is used. Having the set/get/load/sync functions grouped together
makes it easier to get an overview.

REMARK: this commit only defines the class, it is not active in production

Signed-off-by: Jan Iversen <jani@apache.org>

core/settings/qPref.h

@@ -14,6 +14,7 @@
 #include "qPrefGeocoding.h"
 #include "qPrefLanguage.h"
 #include "qPrefLocationService.h"
+#include "qPrefPartialPressureGas.h"
 #include "qPrefProxy.h"
 #include "qPrefTechnicalDetails.h"
 #include "qPrefUnit.h"