Improved acknowledgements: Zabbix service sends a callback when an event
has been acknowledged successfully; then, the acknowledgement menu
button disappears.
Introduced separate list adapter for problems list in MainActivity which
always displays all active problems regardless of severity and host
group.
Refactoring.
Fixed event/problem selection on startup/severity change/host group
change.
First version of Checks landscape layout.
Improved Checks Landscape view. 
Refactoring: list item contents are re-used by using <include>