ui/key,ui/app: introduce ModShift modifier

And add desktop implementations.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/os_windows.go

@@ -88,6 +88,7 @@ const (
 	_USER_TIMER_MINIMUM = 0x0000000A
 
 	_VK_CONTROL = 0x11
+	_VK_SHIFT   = 0x10
 
 	_VK_BACK   = 0x08
 	_VK_DELETE = 0x2e
@@ -271,6 +272,9 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 			if getKeyState(_VK_CONTROL)&0x1000 != 0 {
 				cmd.Modifiers |= key.ModCommand
 			}
+			if getKeyState(_VK_SHIFT)&0x1000 != 0 {
+				cmd.Modifiers |= key.ModShift
+			}
 			w.w.event(cmd)
 		}
 	case _WM_LBUTTONDOWN: