Changes to MainWindowViewModel.cs related to refactor dicussed in Issue #200:

- Renamed method Reload to ReloadFile().
- Removed method Refresh(); replaced it with ReloadFile().
- Change "?" keyboard shortcut to call method ShowHelpDialog().
- Made AddCalendarToTitle() method public.
- Added public method AddNewTask().
- Added public method UpdateTask().
- Added public method DeleteTask().
- Added public method ToggleCompletion().
- Added public method IncreasePriority().
- Added public method DecreasePriority().
- Added public method RemovePriority().
- Added public method PostponeTask().
- Added public method CopySelectedTasksToClipboard().
- Added public method CopySelectedTaskToTextBox().
- Added public method ApplyFilterPreset0().
- Added public method ApplyFilterPreset1().
- Added public method ApplyFilterPreset2().
- Added public method ApplyFilterPreset3().
- Added private method ApplyFilterPreset().
- Reduced size of keyboard shortcut handlers TaskListKeyUp() and TaskListPreviewKeyDown() by calling the newly-added methods.