Implement missing OLEDDisplayUi::setLoadingDrawFunction() in OLEDDisplayUi.cpp

[artkeller]

When I tried to use setLoadingDrawFunction() I realized the missing code in OLEDDisplayUi.cpp.

After adding the 1-liner everything worked fine:

`// -/----- Loading Process ------

void OLEDDisplayUi::setLoadingDrawFunction(LoadingDrawFunction loadingDrawFunction) {
this->loadingDrawFunction = loadingDrawFunction;
}

void OLEDDisplayUi::runLoadingProcess(LoadingStage* stages, uint8_t stagesCount) {
...
`

Thanks for your lib - very useful!

Thomas

[FWeinb]

Thanks for catching this. I added a PR that implements this.