Fixed processing that uses GetAsyncKeyState(). (#505)

Fixed processing that uses GetAsyncKeyState(). (#505)

- Fixed the behaviour in the function CMainFrame::OnDropFiles() that when drag and drop is performed after pressing the control key,
  it behaves as if drag and drop is performed while pressing the control key.
- Fixed the behaviour in the function CImgMergeFrame::OnChildPaneEvent() that when an arrow key is pressed after pressing the shift key or control key,
  it behaves as if the arrow key is pressed while pressing the shift key or control key.