Moving adaptor or proxy instances changes their `this` pointer. But `this` is captured by value in closures used by those instances, and this remains unchanged on move, leading to accessing an invalid instance when a lambda expression executes. Supporting move semantics would require unsubscribing/unregistering vtable, handlers, etc. and re-subscribing and re-registering all that, which is too complicated and may have side effects. Hence it has been decided that these classes are not moveable. One may use an indirection with e.g. `std::unique_ptr` to get move semantics.
Having explicit conversion operator is a good practice according to the C++ core guidelines, as it makes the code safer and better follows the principle of least astonishment. Also, it is consistent with the standard library style, where wrappers like std::variant, std::any, std::optional... also do not provide an implicit conversion to the underlying type. Last but not least, it paves the way for the upcoming std::variant <-> sdbus::Variant implicit conversions without surprising behavior in some edge cases.
This introduces strong types for `std::string`-based D-Bus types. This facilitates safer, less error-prone and more expressive API.
What previously was `auto proxy = createProxy("org.sdbuscpp.concatenator", "/org/sdbuscpp/concatenator");` is now written like `auto proxy = createProxy(ServiceName{"org.sdbuscpp.concatenator"}, ObjectPath{"/org/sdbuscpp/concatenator"});`.
These types are:
* `ObjectPath` type for the object path (the type has been around already but now is also used consistently in sdbus-c++ API for object path strings),
* `InterfaceName` type for D-Bus interface names,
* `BusName` (and its aliases `ServiceName` and `ConnectionName`) type for bus/service/connection names,
* `MemberName` (and its aliases `MethodName`, `SignalName` and `PropertyName`) type for D-Bus method, signal and property names,
* `Signature` type for the D-Bus signature (the type has been around already but now is also used consistently in sdbus-c++ API for signature strings),
* `Error::Name` type for D-Bus error names.
This makes D-Bus proxy signal registration more flexible, more dynamic, and less error-prone since no `finishRegistration()` call is needed. A proxy can register to a signal at any time during its lifetime, and can unregister freely by simply destroying the associated slot.
This improves the D-Bus object API registration/unregistration by making it more flexible, more dynamic, closer to sd-bus API design but still on high abstraction level, and -- most importantly -- less error-prone since no `finishRegistration()` call is needed anymore.
