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/receivecoinsdialog.h

@@ -16,6 +16,7 @@
 #include <QVariant>
 
 class OptionsModel;
+class PlatformStyle;
 class WalletModel;
 
 namespace Ui {
@@ -39,7 +40,7 @@ public:
         MINIMUM_COLUMN_WIDTH = 130
     };
 
-    explicit ReceiveCoinsDialog(QWidget *parent = 0);
+    explicit ReceiveCoinsDialog(const PlatformStyle *platformStyle, QWidget *parent = 0);
     ~ReceiveCoinsDialog();
 
     void setModel(WalletModel *model);
@@ -57,6 +58,8 @@ private:
     GUIUtil::TableViewLastColumnResizingFixer *columnResizingFixer;
     WalletModel *model;
     QMenu *contextMenu;
+    const PlatformStyle *platformStyle;
+
     void copyColumnToClipboard(int column);
     virtual void resizeEvent(QResizeEvent *event);