Change the way Twitch accounts are stored in AccountManager

This is done in a way which should simplify abstracting it to other
types of accounts if needed in the future

Remove comment about removing singletons - we're keeping them (and probably restoring some)

IrcManager now updates its "account" reference automatically through the
AccountManager.Twitch.userChanged-signal

Remove unused IrcManager getUser-method

IrcManager::beginConnecting is no longer called asynchronously. This
might want to be reverted in a more controlled asynchronous manner.

User Accounts are now stored as Shared Pointers instead of using
references/copies everywhere

src/application.cpp

@@ -18,7 +18,6 @@ Application::Application()
     , channelManager(this->windowManager, this->emoteManager, this->ircManager)
     , ircManager(this->channelManager, this->resources, this->emoteManager, this->windowManager)
 {
-    // TODO(pajlada): Get rid of all singletons
     logging::init();
     SettingsManager::getInstance().load();
 
@@ -29,8 +28,6 @@ Application::Application()
 
     AccountManager::getInstance().load();
 
-    this->ircManager.setUser(AccountManager::getInstance().getTwitchUser());
-
     // XXX
     SettingsManager::getInstance().updateWordTypeMask();