Remove redundant 'if' statement
Add 'HandleClose' class

This class is wrapper of 'HANDLE' which should be closed by 'CloseHandle'.
Change access level of non-const 'GetColors'
Change behavior of 'GetColors'

When there is no color table, return 'NULL'.
Change behavior of 'SetColors'

Check validity and return whether successful.
Add constructor from 'BITMAPINFOHEADER' to 'BitmapInfoHeader'

This constructs '*this' directly.
Add constructor from 'BitmapInfoHeader' to 'BitmapInfo'
Add constructor from 'BitmapInfoHeader' to 'BitmapInfoPtr'
Add 'Clone' to 'BitmapInfoPtr'
Improve impl of 'DIBitmap' ctor
Add ctor from 'BitmapInfoPtr' to 'DIBitmap'