Merge bitcoin-core/gui#330: Allow prompt icon to be colorized

2f23ad2c40 qt: allow prompt icon to be colorized (Jarol Rodriguez)

Pull request description:

  Opening the console on macOS, while in dark mode, the console prompt icon will not be colorized white like other icons. This applies the `platformStyle` to the icon so that It can be colorized white.

  While here, refactor the `promptIcon` widget from a `QPushButton` to `QLabel`; which is more appropriate, per [Qt Docs](https://doc.qt.io/qt-5/qlabel.html#details):
  > QLabel is used for displaying text or an image. No user interaction functionality is provided.

  | Master        | PR               |
  | ----------- | ----------- |
  | ![Screen Shot 2021-05-14 at 11 46 33 PM](https://user-images.githubusercontent.com/23396902/118347462-8f689780-b511-11eb-8335-329f7d2a9992.png) | ![Screen Shot 2021-05-14 at 11 45 41 PM](https://user-images.githubusercontent.com/23396902/118347463-92638800-b511-11eb-9044-073f51ef27ff.png) |

ACKs for top commit:
  hebasto:
    ACK 2f23ad2c40

Tree-SHA512: 21f8b1610e4820c9064bbd08608b5467e5b9499e2a3b149ff223e37b60e7d560497255c733eafa5434628a84b9f7b7c91d8b0f34b02be2f9ceb3ab21a4d555a8

src/qt/rpcconsole.cpp

@@ -529,6 +529,8 @@ RPCConsole::RPCConsole(interfaces::Node& node, const PlatformStyle *_platformSty
     //: Secondary shortcut to decrease the RPC console font size.
     GUIUtil::AddButtonShortcut(ui->fontSmallerButton, tr("Ctrl+_"));
 
+    ui->promptIcon->setIcon(platformStyle->SingleColorIcon(QStringLiteral(":/icons/prompticon")));
+
     // Install event filter for up and down arrow
     ui->lineEdit->installEventFilter(this);
     ui->lineEdit->setMaxLength(16 * 1024 * 1024);