qt: Introduce PlatformStyle

Introduce a PlatformStyle to handle platform-specific customization of
the UI.

This replaces 'scicon', as well as #ifdefs to determine whether to place
icons on buttons.

The selected PlatformStyle defaults to the platform that the application
was compiled on, but can be overridden from the command line with
`-uiplatform=<x>`.

Also fixes the warning from #6328.

src/qt/transactiontablemodel.cpp

@@ -8,7 +8,7 @@
 #include "guiconstants.h"
 #include "guiutil.h"
 #include "optionsmodel.h"
-#include "scicon.h"
+#include "platformstyle.h"
 #include "transactiondesc.h"
 #include "transactionrecord.h"
 #include "walletmodel.h"
@@ -222,12 +222,13 @@ public:
     }
 };
 
-TransactionTableModel::TransactionTableModel(CWallet* wallet, WalletModel *parent):
+TransactionTableModel::TransactionTableModel(const PlatformStyle *platformStyle, CWallet* wallet, WalletModel *parent):
         QAbstractTableModel(parent),
         wallet(wallet),
         walletModel(parent),
         priv(new TransactionTablePriv(wallet, this)),
-        fProcessingQueuedTransactions(false)
+        fProcessingQueuedTransactions(false),
+        platformStyle(platformStyle)
 {
     columns << QString() << QString() << tr("Date") << tr("Type") << tr("Label") << BitcoinUnits::getAmountColumnTitle(walletModel->getOptionsModel()->getDisplayUnit());
     priv->refreshWallet();
@@ -521,7 +522,7 @@ QVariant TransactionTableModel::data(const QModelIndex &index, int role) const
     case Qt::DecorationRole:
     {
         QIcon icon = qvariant_cast<QIcon>(index.data(RawDecorationRole));
-        return TextColorIcon(icon);
+        return platformStyle->TextColorIcon(icon);
     }
     case Qt::DisplayRole:
         switch(index.column())