refactor: remove <util/system.h> include from wallet.h

Since we no longer store a ref to the global `ArgsManager`
inside the wallet, we can move the util/system.h
include to the cpp.

This dependency removal opened a can of worms, as few
other places were, invalidly, depending on the wallet's
header including it.

src/qt/optionsmodel.cpp

@@ -18,6 +18,7 @@
 #include <netbase.h>
 #include <txdb.h>       // for -dbcache defaults
 #include <util/string.h>
+#include <util/system.h>
 #include <validation.h> // For DEFAULT_SCRIPTCHECK_THREADS
 #include <wallet/wallet.h> // For DEFAULT_SPEND_ZEROCONF_CHANGE
 
@@ -640,6 +641,11 @@ bool OptionsModel::isRestartRequired() const
     return settings.value("fRestartRequired", false).toBool();
 }
 
+bool OptionsModel::hasSigner()
+{
+    return gArgs.GetArg("-signer", "") != "";
+}
+
 void OptionsModel::checkAndMigrate()
 {
     // Migration of default values