Changes related to major refactor referenced in Issue #200.

- Added Edit menu. Closes Issue #217
- Added Task menu. Closes Issue #205.
- Expanded Filter menu to show the apply filter commands.
- Added InputGestureText to menu items with keyboard shortcuts.
- Added additional keyboard shotcuts such as Ctrl+N (new file) and Ctrl+O (open file) and F10 (Options).
- Refactored event handlers for menu items and keyboard shortcuts to be RoutedUICommands and relat4ed key and mouse bindings defined in MainWindow.xaml. Removes two keyboard event handling methods in MainWindowViewModel.cs. Related to refactor discussed in Issue #200.