Generalize task related models

.../cz/muni/fi/pv168/project/model/db/AbstractDataModel.java

.../java/cz/muni/fi/pv168/project/model/db/SubTaskModel.java

.../cz/muni/fi/pv168/project/model/db/TaskCategoryModel.java

...z/muni/fi/pv168/project/model/db/TaskDependencyModel.java

...ain/java/cz/muni/fi/pv168/project/model/db/TaskModel.java

.../fi/pv168/project/model/db/TaskPropertyListDataModel.java

...cz/muni/fi/pv168/project/model/db/UpdatableDataModel.java

...pv168/project/ui/main/view/CategoryAndStatisticsView.java

...a/cz/muni/fi/pv168/project/ui/main/view/CategoryView.java

...cz/muni/fi/pv168/project/ui/main/view/StatisticsView.java