Merge #150: Temporary dark mode button

c231381 qml: temporary dark mode button (jarolrod)

Pull request description:

  A temporary placement for a button that toggles dark mode on and off within the entire UI. Necessary to visually ensure the UI is looking the way that the designers intend. For example, this rebased on top of #149:

  ![current-ui-darkmode](https://user-images.githubusercontent.com/23396902/184046910-953acfdc-c72c-4f34-956a-a7575e2f7c9e.png)

  [![Windows](https://img.shields.io/badge/OS-Windows-green)](https://api.cirrus-ci.com/v1/artifact/github/bitcoin-core/gui-qml/win64/insecure_win_gui.zip?branch=pull/150)
  [![Intel macOS](https://img.shields.io/badge/OS-Intel%20macOS-green)](https://api.cirrus-ci.com/v1/artifact/github/bitcoin-core/gui-qml/macos/insecure_mac_gui.zip?branch=pull/150)
  [![Apple Silicon macOS](https://img.shields.io/badge/OS-Apple%20Silicon%20macOS-green)](https://api.cirrus-ci.com/v1/artifact/github/bitcoin-core/gui-qml/macos_arm64/insecure_mac_arm64_gui.zip?branch=pull/150)
  [![ARM64 Android](https://img.shields.io/badge/OS-Android-green)](https://api.cirrus-ci.com/v1/artifact/github/bitcoin-core/gui-qml/android/insecure_android_apk.zip?branch=pull/150)

ACKs for top commit:
  hebasto:
    ACK c231381, tested on Ubuntu 22.04.

Tree-SHA512: a8d761801961b8f0e60a09c41504a77c328ea25a0c64f937a9f8609b2741a1cba8428ce0c092ffc704e19b7ec385a4d380e99c719ef62c466fb63b85ae5e4831

Author: hebasto

src/qml/components/DeveloperOptions.qml

@@ -29,6 +29,13 @@ ColumnLayout {
             description: qsTr("Mainnet")
         }
     }
+    Setting {
+        Layout.fillWidth: true
+        header: qsTr("Dark Mode")
+        actionItem: OptionSwitch {
+            onToggled: Theme.toggleDark()
+        }
+    }
     Setting {
         Layout.fillWidth: true
         header: qsTr("Other option...")