Add keyboard input modifier Command.

src/Avalonia.Input/IKeyboardDevice.cs

@@ -14,9 +14,10 @@ namespace Avalonia.Input
         Control = 2,
         Shift = 4,
         Windows = 8,
-        LeftMouseButton = 16,
-        RightMouseButton = 32,
-        MiddleMouseButton = 64
+        Command = 16, // Special case CMD on OSX, CTRL on other platforms.
+        LeftMouseButton = 32,
+        RightMouseButton = 64,
+        MiddleMouseButton = 128
     }
 
     [Flags]

src/OSX/Avalonia.MonoMac/TopLevelImpl.cs

@@ -224,13 +224,16 @@ namespace Avalonia.MonoMac
             {
                 var rv = new InputModifiers();
                 if (mod.HasFlag(NSEventModifierMask.ControlKeyMask))
-                    rv |= InputModifiers.Control;
+                   {rv |= InputModifiers.Control;
                 if (mod.HasFlag(NSEventModifierMask.ShiftKeyMask))
                     rv |= InputModifiers.Shift;
                 if (mod.HasFlag(NSEventModifierMask.AlternateKeyMask))
                     rv |= InputModifiers.Alt;
                 if (mod.HasFlag(NSEventModifierMask.CommandKeyMask))
+                {
                     rv |= InputModifiers.Windows;
+                    rv = InputModifiers.Command;
+                }
 
                 if (_isLeftPressed)
                     rv |= InputModifiers.LeftMouseButton;

src/Windows/Avalonia.Win32.Interop/Wpf/WpfTopLevelImpl.cs

@@ -151,7 +151,10 @@ namespace Avalonia.Win32.Interop.Wpf
             if (state.HasFlag(ModifierKeys.Alt))
                 rv |= InputModifiers.Alt;
             if (state.HasFlag(ModifierKeys.Control))
+            {
                 rv |= InputModifiers.Control;
+                rv |= InputModifiers.Command;
+            }
             if (state.HasFlag(ModifierKeys.Shift))
                 rv |= InputModifiers.Shift;
             //TODO: mouse modifiers

src/Windows/Avalonia.Win32/Input/WindowsKeyboardDevice.cs

@@ -29,6 +29,7 @@ namespace Avalonia.Win32.Input
                 if (IsDown(Key.LeftCtrl) || IsDown(Key.RightCtrl))
                 {
                     result |= InputModifiers.Control;
+                    result |= InputModifiers.Command;
                 }
 
                 if (IsDown(Key.LeftShift) || IsDown(Key.RightShift))

src/Windows/Avalonia.Win32/OleDropTarget.cs

@@ -59,7 +59,10 @@ namespace Avalonia.Win32
             if (state.HasFlag(UnmanagedMethods.ModifierKeys.MK_SHIFT))
                 modifiers |= InputModifiers.Shift;
             if (state.HasFlag(UnmanagedMethods.ModifierKeys.MK_CONTROL))
+            {
                 modifiers |= InputModifiers.Control;
+                modifiers |= InputModifiers.Command;
+            }
             if (state.HasFlag(UnmanagedMethods.ModifierKeys.MK_ALT))
                 modifiers |= InputModifiers.Alt;
             return modifiers;