Per-shell-profile keyboard shortcuts (#85)

* Added new command wrapper, abstracts static and dynamic commands

- All uses of the `Command` are replaced with either `AppCommand`, for a command that is static to the scope of the application, or a reference to an ICommand (specifically, an `EnumCommand<AppCommand>()` instance), which provides the abstract wrapper.
- Functionality seems intact.
- Next step is to have the MainViewModel add the per-shell commands via SettingsService.CreateShellKeyboardShortcuts(), and have the SettingsService.ParseCommandString() function parse the command strings passed around properly.
- Finally, add the keyboard chord input to each shell's settings page.

* First kick at adding a new terminal keyboard command.

- It isn't presented anywhere, but references to keybindings should be using the ICommand key as their identifier for recognizing anything, and when xterm passes back a keyboard command event, the SettingsService is asked to convert the string into an ICommand.

* Renaming ICommand to Command.

It's not longer an interface, so this makes more sense.

* Renaming back to Command, and new class is AbstractCommand.

Going to simplify the PR diff considerably.

* Shortcuts for default shells work. Need to get the shell profile edit UI elements working.

* Added keyboard shortcut UI elements to shell profile pages.

* New and deleted shell profiles propagate to all terminal views.

* A bit of cleanup.

* Camel-casing command text to fix Ctrl+C and copy behaviour.

* Fixed failing test.

The new on disk store uses a .ToString(), not the Enum description, since the command isn't an Enum, it's a class now in general, and those semantics make more sense.

* Adding default value for null keybindings in shell profiles.

* Merge branch 'shell_commands' of https://github.com/Riebart/FluentTerminal into pr/85

Improved default value for keybinding property

* Renamed KeyBinding collection to KeyBindings

* Use strings as Command parameter of KeyBinding

* Added missing Commands to keyBindings of default shellProfiles

* fix build and unit tests after merge with master

* improve UI

* Split OnShellProfileCollectionChanged event

FluentTerminal.App.ViewModels/Settings/KeyBindingViewModel.cs

@@ -1,7 +1,7 @@
 using FluentTerminal.App.Services;
+using FluentTerminal.App.ViewModels.Infrastructure;
 using FluentTerminal.Models;
 using GalaSoft.MvvmLight;
-using GalaSoft.MvvmLight.Command;
 using System;
 using System.Threading.Tasks;
 
@@ -11,18 +11,21 @@ namespace FluentTerminal.App.ViewModels.Settings
     {
         private readonly IDialogService _dialogService;
 
-        public KeyBindingViewModel(KeyBinding keyBinding, IDialogService dialogService)
+        public KeyBindingViewModel(KeyBinding keyBinding, IDialogService dialogService, KeyBindingsViewModel parent)
         {
             Model = keyBinding;
+            Parent = parent;
             _dialogService = dialogService;
-            EditCommand = new RelayCommand(async () => await Edit().ConfigureAwait(false));
-            DeleteCommand = new RelayCommand(async () => await Delete().ConfigureAwait(false));
+            EditCommand = new AsyncCommand(Edit);
+            DeleteCommand = new AsyncCommand(Delete);
         }
 
         public event EventHandler Deleted;
 
         public event EventHandler Edited;
 
+        public KeyBindingsViewModel Parent { get; }
+
         public bool Meta
         {
             get => Model.Meta;
@@ -62,8 +65,8 @@ namespace FluentTerminal.App.ViewModels.Settings
             }
         }
 
-        public RelayCommand DeleteCommand { get; }
-        public RelayCommand EditCommand { get; }
+        public IAsyncCommand DeleteCommand { get; }
+        public IAsyncCommand EditCommand { get; }
 
         public int Key
         {