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.h

@@ -10,6 +10,7 @@
 #include <QAbstractTableModel>
 #include <QStringList>
 
+class PlatformStyle;
 class TransactionRecord;
 class TransactionTablePriv;
 class WalletModel;
@@ -23,7 +24,7 @@ class TransactionTableModel : public QAbstractTableModel
     Q_OBJECT
 
 public:
-    explicit TransactionTableModel(CWallet* wallet, WalletModel *parent = 0);
+    explicit TransactionTableModel(const PlatformStyle *platformStyle, CWallet* wallet, WalletModel *parent = 0);
     ~TransactionTableModel();
 
     enum ColumnIndex {
@@ -82,6 +83,7 @@ private:
     QStringList columns;
     TransactionTablePriv *priv;
     bool fProcessingQueuedTransactions;
+    const PlatformStyle *platformStyle;
 
     void subscribeToCoreSignals();
     void unsubscribeFromCoreSignals();