Notify mod installation to plugins after directory structure is ready.

[Holt59]

This is actually a pretty small change that add the following before calling refresh() (and remove the call to notifyModInstalled afterward):

connect(this, &OrganizerCore::directoryStructureReady, this, [=] {
  const int modIndex = ModInfo::getIndex(modName);
  if (modIndex != UINT_MAX) {
    const auto modInfo = ModInfo::getByIndex(modIndex);
    m_ModList.notifyModInstalled(modInfo.get());
  }
  }, Qt::SingleShotConnection);

Most of the changes are trying to have a single function called by both installDownload() and installArchive() instead of having two almost identical functions.