Add ability to restrict characteristic and descriptor writes before execution

Currently QLowEnergyController in peripheral mode emits characteristicChanged() and descriptorWritten() signals when the char or desc was changed by the client/central device. There is no way to process the write request before it is being accepted by the peripheral. Android seems to be able to do this as part of callback handlers.

A way of implementing this might be to pass a std::function parameter to the QLowEnergyCharacteristicData & QLowEnergyDescriptorData object which acts as call back.