core: create qPrefUnit 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/CMakeLists.txt

@@ -110,6 +110,7 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	settings/qPrefFacebook.cpp
 	settings/qPrefPrivate.cpp
 	settings/qPrefProxy.cpp
+	settings/qPrefUnit.cpp
 
 	#Subsurface Qt have the Subsurface structs QObjectified for easy access via QML.
 	subsurface-qt/DiveObjectHelper.cpp