The exported object with the properties
A quark representing the D-Bus interface name
A quark representing the D-Bus property name
A GValue pre-initialized to the right type, into which to put the value
The getter_data from the #TpDBusPropertiesMixinPropImpl
Signature of a callback used to get the value of a property.
For simplicity, in this mixin we don't allow getting a property to fail; implementations must always be prepared to return something.