Use Qt signal for macOS Dock icon click event

This moves the Dock icon click reaction code to the common place and
allows some cleanup in obj_c code.

According to the Apple's docs `class_replaceMethod` behaves as
`class_addMethod`, if the method identified by name does not yet exist;
or as `method_setImplementation`, if it does exist.

Github-Pull: #14597
Rebased-From: 2464925e7b

src/qt/bitcoingui.cpp

@@ -773,6 +773,12 @@ void BitcoinGUI::macosDockIconActivated()
     show();
     activateWindow();
 }
+#else
+void BitcoinGUI::macosDockIconActivated()
+{
+    show();
+    activateWindow();
+}
 #endif
 
 void BitcoinGUI::optionsClicked()

src/qt/macdockiconhandler.mm

@@ -49,5 +49,4 @@ void MacDockIconHandler::cleanup()
  */
 void ForceActivation()
 {
-    [[NSApplication sharedApplication] activateIgnoringOtherApps:YES];
-}
+    [[NSApplication sharedApplication] activateIgnoringOtherApps:YES];