* DPL MQTT handler: modernize
* there is no need to tokenize and check the topic of a received MQTT
message if we only subscribe to a single topic. all messages will be
for that topic. avoid testing the topic in the callback alltogether.
* use std::string and std::stoi over allocating and deleting a buffer
and copying charactes around.
* use a switch statement to process the actual payload.
* break a long line.
* DPL: fix getMode() return value
getMode() returned a bool. probably its return type was not adjusted
when the third mode was introduced. this lead to mode 2 being cast to
true implicitly, which in turn was used to construct a String, such that
"1" was published as the DPL mode when in fact it was 2.
make the mode an enum class to avoid such problems in the future.
inline getMode() and setMode().
fix indention.
