Fixed issue #179 - appearance of drop indicators then Windows option "Show window contents while dragging" is

FloatingDragPreview.cpp: moved code from moveEvent into moveFloating function to remove indirection and to simplify code
Moved code from moveEvent() function into moveFloating() to remove indirection and to simplify code
Implemented Windows drag handling with native WM_ nonclient area messages

Author: githubuser0xFFFF