unified VERSION definition, TRACE() macro

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
put BasicWriter implementation into separate class

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[statefspp] read/write property wrapper

RWProperty template to wrap read-only properties to make them
writable. DiscreteWritable and AnalogWriteable provide traits to
construct implementation based on corresponding RWProperty
instantiation. statefspp example is updated to demostrate the usage of
the wrapper.

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
statefspp example: more docs and some refactoring

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
converted enum PropertyStatus to the class

To avoid subtle issues due to implicit integer conversion

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[service] using default permissions control

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>