Bug in calculateGuessQuestionWinner method corrected.

# Conflicts:
#	KvizClient/app/src/main/java/onlab/kvizclient/LobbyActivity.java

Server user can restrict the list of questions from the full database to a limited number of random questions with selecting which topics to include and providing the maximum number of questions per topic.

Now the saved database files can be opened correctly.

Also added "currentIndex" member variable to Question class to be able to store the current questions list (not only the full database).

Question database is persistent until a new question file is loaded.

QuestionListFragment added which displays the list of all questions in the database and the server user can select what question to be next in the game. In GameActivity the GameControlFragment and the QuestionListFragment are displayed by a ViewPager so the server user can alternate between them.

GameControlFragment added.
Gradle version is modified. (Because of updating Android Studio.)

GuessQuestionFragment and AnsweredQuestionFragment added.

MultipleChoiceFragment can handle answers of more than two players.

In QuizUp mode questions are no more sent by pressing a button by server user. They are sent automatically with timer.

If a time limit is set, a progress bar shows the remaining time and the fragment lets answering only during this limited time.

In Server/LobbyActivity communication thread is also closed with a boolean variable when new activity is started.

At the end of the game, QuizUpActivity displays the outcome to the players.

Closing of the LobbyActivity's CommunicationThread with dummy messages sent by GameActivity.

Merge branch 'ClientGameActivity' of https://git.sch.bme.hu/KGregPFerenc/AndroidOnlabKviz2018 into ClientGameActivity

The name of the chosen file is displayed and a default file is added if no file is chosen manually.