add member alt for arg_keyboard

source/gui/detail/bedrock_posix.cpp

@@ -223,6 +223,7 @@ namespace detail
 	{
 		XKeyEvent xkey;
 		nana::detail::platform_spec::instance().read_keystate(xkey);
+		arg.alt = (xkey.state & Mod1Mask);
 		arg.ctrl = (xkey.state & ControlMask);
 		arg.shift = (xkey.state & ShiftMask);
 	}

source/gui/detail/bedrock_windows.cpp

@@ -1622,6 +1622,7 @@ namespace detail
 
 	void bedrock::get_key_state(arg_keyboard& kb)
 	{
+		kb.alt = (0 != (::GetKeyState(VK_MENU) & 0x80));
 		kb.ctrl = (0 != (::GetKeyState(VK_CONTROL) & 0x80));
 		kb.shift = (0 != (::GetKeyState(VK_SHIFT) & 0x80));
 	}