macOS Mojave dark mode support

[christophe-calmejane]

Dark mode not properly supported. There is a fix in Qt, but we'll have to wait for next Qt update. In the meantime, we have some options:

• Try to use a style (but for the whole application)
• Disable it from the plist (NSRequiresAquaSystemAppearance=true)

One can disable dark mode manually with the following command: defaults write fr.KikiSoft.Hive NSRequiresAquaSystemAppearance -bool yes

[christophe-calmejane]

Forced light mode in 27207afa until full support in Qt