Add an Info.plist key to enable support for dark mode in Mojave

Mojave uses tis key to determine how to draw the titlebar.
See https://developer.apple.com/documentation/appkit/nsappearancecustomization/choosing_a_specific_appearance_for_your_app?language=objc

setup.py

@@ -669,6 +669,8 @@ def src_ignore(parent, entries):
             LSMinimumSystemVersion='10.12.0',
             LSRequiresNativeExecution=True,
             NSAppleScriptEnabled=False,
+            # Needed for dark mode in Mojave when linking against older SDKs
+            NSRequiresAquaSystemAppearance='NO',
             NSHumanReadableCopyright=time.strftime(
                 'Copyright %Y, Kovid Goyal'),
             CFBundleGetInfoString='kitty, an OpenGL based terminal emulator https://sw.kovidgoyal.net/kitty',