This improves usability of sdbus-c++ in downstream CMake projects. CMake options now have `SDBUSCPP_` prefix so potential conflicts with downstream options/variables are minimized. Also, all configurable options and variables are placed in a single place in the root CMake file.
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.
