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/qPrefPrivate.h

@@ -23,6 +23,7 @@ public:
 	friend class qPrefGeocoding;
 	friend class qPrefLanguage;
 	friend class qPrefLocationService;
+	friend class qPrefPartialPressureGas;
 	friend class qPrefProxy;
 	friend class qPrefTechnicalDetails;
 	friend class qPrefUnits;