Add icon for when no default account is selected

Also fixed a bug where the icon on the accounts button wouldn't change
when the user checked the "Use as default" checkbox in the account
selection dialog.

logic/lists/MojangAccountList.h

@@ -133,6 +133,11 @@ signals:
 	 */
 	void listChanged();
 
+	/*!
+	 * Signal emitted to indicate that the active account has changed.
+	 */
+	void activeAccountChanged();
+
 protected:
 	/*!
 	 * Called whenever the list changes.
@@ -140,6 +145,12 @@ protected:
 	 */
 	void onListChanged();
 
+	/*!
+	 * Called whenever the active account changes.
+	 * Emits the activeAccountChanged() signal and autosaves the list if enabled.
+	 */
+	void onActiveChanged();
+
 	QList<MojangAccountPtr> m_accounts;
 
 	/*!