common: Provide helper for resolving overloaded function pointers

common: Provide helper for resolving overloaded function pointers

The pointer-to-member-function connect syntax cannot directly deal
with overloaded method signatures. Provide a helper to avoid ugly
static_cast incantations:

  #include "util.h"
  connect(this, selectOverload<int, QString>(&MyClass::mySignal), other, &Other::mySlot);

This helper can be used for both signals and slots.