qt: Add Dark Mode [Closed due to incorrect rebase] Reopened in !6131

Reopened in !6131 (merged)

Added Native Dark Mode 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

PREVIEWS: