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/intro.cpp

@@ -6,7 +6,6 @@
 #include "ui_intro.h"
 
 #include "guiutil.h"
-#include "scicon.h"
 
 #include "util.h"
 
@@ -168,7 +167,7 @@ void Intro::pickDataDirectory()
         /* If current default data directory does not exist, let the user choose one */
         Intro intro;
         intro.setDataDirectory(dataDir);
-        intro.setWindowIcon(SingleColorIcon(":icons/bitcoin"));
+        intro.setWindowIcon(QIcon(":icons/bitcoin"));
 
         while(true)
         {