Fixed possible NullPointerExceptions.
Moved choice between Mock and real data from MainActivity to
BaseActivity.
Preferences: Moved separate screens to categories in the main screen.
Events/Problems: Simplified loading bar business, extracted list pager
adapter (for severity tabs).
Moved list pager adapters to the service (in order to keep the state
(selected severity) in the service instead of the activities/fragments).
Fixed a problem with the Tab page indicator.
Position of the host group spinner (Events/Problems/Checks) is
maintained in the service now. -> Changing activities does not reset the
current host group.
Host groups and hosts are now loaded from Zabbix during the login
process because they are important at many points in the application.
Events/Problems: Moved position information to the adapter (removing
duplicate status variables in activities/fragments).
Preferences: If the Zabbix server name or user name has been changed,
all data is cleared and refreshed.
Improved error handling.