settings: add a callback to notify user of a settings change

[jcelerier]

This will allow users to create DMIs that won't require reboots

[cbicari]

from template review branch :
in [basic-osc/src/main.cpp]
OSCMessage msg1(("/" + puara.dmi_name()).c_str());
msg1.add(sensor);
Udp.beginPacket(puara.IP1().c_str(), puara.PORT1());
Udp.beginPacket(oscIP_1.c_str(), oscPort_1);

Since in this example we are reloading the ip / port on each loop() tick, it wouldn't be slower to just do

auto ip = puara.getVarText("oscIP");
auto port = puara.getVarNumber("oscPort");
if(!ip.empty() && ip != "0.0.0.0") {
..
Udp.beginPacket(ip.c_str(), port);
}

this way updates to the web ui would be taken into account directly

[jcelerier]

About what I said in the above comment: this was in the case where we only have simple logic.

If we have a very basic system that just sends OSC messages, then it's going to be exactly as performant to just re-read the OSC address & port on each loop() as they just get read from a local cache.

If we have something more advanced, for instance not just a OSC client but also an OSC server, then it is necessary to have the callback so that we can stop & restart the server with the correct new port, if the user changed it as part of the settings.

Cheers!

[jcelerier]

TODO: rebase when #38 is merged so that we can check whether this doesn't break CI

[cbicari]

compiles all tests :)
could test with templates too