This commit adds to our UI the support for macOS 10.14+ Dark Mode.
Qt already adapts a large fraction of the color scheme, but some
label colors had to be adjusted and were put in separate STYLE_DARK
constants. To determine if the OS is set in Dark or Light Mode, we
use a new dependency, included in the vendor folder:
Darkdetect - license: BSD-3-Clause
To allow the app bundle to use the Dark Mode APIs, a constant is
added in the info.plist ('NSRequiresAquaSystemAppearance': False)
