Support Dark Mode

Most of 'modern' applications supports application controlled/configured dark mode, such as Notepad++, Eclipse, Intellij IDEA. Notepad Next should support dark mode too.

Qt dark mode support for Windows has some issues but there are third party Qt dark mode support such as QDarkStyleSheet: https://github.com/ColinDuquesnoy/QDarkStyleSheet. I have done some investigation on QDarkStyleSheet, it seems possible to implement dark mode based on QDarkStyleSheet. I have tested it on Linux and the result is positive (I have trouble to install Qt on Windows and can't build Notepad Next on/for Windows, proxy issue).

I can contribute on the dark mode implementation