qt: Move BitcoinGUI initializers to class, fix initializer order warning

- C++11-ize the code (move initializers to class, change 0 to `nullptr` where appropriate)
- Make sure `m_wallet_selector` is initialized
- And fix the following warning:

    bitcoin/src/qt/bitcoingui.cpp:122:5⚠️ field 'spinnerFrame' will be initialized after field 'm_wallet_selector_label' [-Wreorder]
        spinnerFrame(0),

src/qt/bitcoingui.cpp

@@ -76,51 +76,7 @@ const std::string BitcoinGUI::DEFAULT_UIPLATFORM =
 
 BitcoinGUI::BitcoinGUI(interfaces::Node& node, const PlatformStyle *_platformStyle, const NetworkStyle *networkStyle, QWidget *parent) :
     QMainWindow(parent),
-    enableWallet(false),
     m_node(node),
-    clientModel(0),
-    walletFrame(0),
-    unitDisplayControl(0),
-    labelWalletEncryptionIcon(0),
-    labelWalletHDStatusIcon(0),
-    labelProxyIcon(0),
-    connectionsControl(0),
-    labelBlocksIcon(0),
-    progressBarLabel(0),
-    progressBar(0),
-    progressDialog(0),
-    appMenuBar(0),
-    appToolBar(0),
-    overviewAction(0),
-    historyAction(0),
-    quitAction(0),
-    sendCoinsAction(0),
-    sendCoinsMenuAction(0),
-    usedSendingAddressesAction(0),
-    usedReceivingAddressesAction(0),
-    signMessageAction(0),
-    verifyMessageAction(0),
-    aboutAction(0),
-    receiveCoinsAction(0),
-    receiveCoinsMenuAction(0),
-    optionsAction(0),
-    toggleHideAction(0),
-    encryptWalletAction(0),
-    backupWalletAction(0),
-    changePassphraseAction(0),
-    aboutQtAction(0),
-    openRPCConsoleAction(0),
-    openAction(0),
-    showHelpMessageAction(0),
-    trayIcon(0),
-    trayIconMenu(0),
-    notificator(0),
-    rpcConsole(0),
-    helpMessageDialog(0),
-    modalOverlay(0),
-    prevBlocks(0),
-    spinnerFrame(0),
-    m_wallet_selector_label(nullptr),
     platformStyle(_platformStyle)
 {
     QSettings settings;