[3.0.x] qt: add new option to use a dark palette

OLD MERGE REQUEST: !5932 (closed)

Added Dark Palette For VLC 3.0

• Added checkbox in Preferences to switch to dark mode. (sprefs-interface.ui)
• Added a new function for Windows so a dark title-bar is enabled when using dark mode in qvlcframe.hpp.
• Modified about.ui file and removed custom stylesheet. Now the stylesheet is applied in the constructor of the About Dialog based on the mode (help.cpp).
• Dark mode works best with fusion. It is applied dynamically but requires restart to work properly.

DARK TITLEBAR FOR WINDOWS

• This is implemented in the files qvlcframe.hpp.
• It uses DwmSetWindowAttribute Function to from the dwmapi.dll to apply dark titlebar.
• Requires Restart.

Enabled the StyleComboBox For Windows:

• Fusion
• Windows
• WindowsXP
• WindowsVista

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