Merge #14593: gui: explicitly disable "Dark Mode" appearance on macOS

cf2f430 gui: explicitly disable "Dark Mode" appearance on macOS (fanquake)

Pull request description:

  Bitcoin Core's gui is pretty broken for a someone using macOS in "Dark Mode"; the biggest issue being lots of white text on a white background, leaving most inputs, the rpc console etc unusable.

  This is likely something we'll have to wait for Qt to fix/support, so it's probably easiest to just disable for now, rather than provide a broken UI; see screenshots below. This issue might not have come up much yet, given the issues with the 0.17.0 dmg.

  Apple documentation on "Opting out of Dark Mode" is [here](https://developer.apple.com/documentation/appkit/nsappearancecustomization/choosing_a_specific_appearance_for_your_app).
  > If you need extra time to work on your app's Dark Mode support, you can temporarily opt out by including the NSRequiresAquaSystemAppearance key (with a value of true) in your app’s Info.plist file. Setting this key to true causes the system to ignore the user's preference and always apply a light appearance to your app.

  Related upstream tickets:
  * [Qt Applications are not usable under macOS Mojave with dark theme](https://bugreports.qt.io/browse/QTBUG-68850)
  * [macOS 10.14 Mojave Support](https://bugreports.qt.io/browse/QTBUG-68891)

  Screenshots:
  ![main](https://user-images.githubusercontent.com/863730/47616668-f8769400-dafa-11e8-9ec5-9f184d8210f8.png)
  ![pay to](https://user-images.githubusercontent.com/863730/47616670-fad8ee00-dafa-11e8-876f-c656de0e31c4.png)
  ![qr code - not hilighted](https://user-images.githubusercontent.com/863730/47616671-fdd3de80-dafa-11e8-873f-f7fa3e91c2a0.png)
  ![qr code](https://user-images.githubusercontent.com/863730/47616672-00363880-dafb-11e8-883c-59a084b18120.png)

Tree-SHA512: 01f6f6b1fed0f20937b433adef38ff55ccacce8b4405dd6be67356ee9a470a56b8e511cc79909a39c3091a852f444284e49a8773cf6c037a29669804185ca92d

Author: laanwj

share/qt/Info.plist.in

@@ -99,6 +99,9 @@
 
   <key>LSAppNapIsDisabled</key>
     <string>True</string>
+
+  <key>NSRequiresAquaSystemAppearance</key>
+    <string>True</string>
 
   <key>LSApplicationCategoryType</key>
     <string>public.app-category.finance</string>