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/walletmodel.h

@@ -17,6 +17,7 @@
 
 class AddressTableModel;
 class OptionsModel;
+class PlatformStyle;
 class RecentRequestsTableModel;
 class TransactionTableModel;
 class WalletModelTransaction;
@@ -100,7 +101,7 @@ class WalletModel : public QObject
     Q_OBJECT
 
 public:
-    explicit WalletModel(CWallet *wallet, OptionsModel *optionsModel, QObject *parent = 0);
+    explicit WalletModel(const PlatformStyle *platformStyle, CWallet *wallet, OptionsModel *optionsModel, QObject *parent = 0);
     ~WalletModel();
 
     enum StatusCode // Returned by sendCoins