Make Events for snapshots always available, even when PNG support is disabled.

- otherwise, builds with and without PNG support will potentially have different mappings
- this makes it very hard to test on devel systems where support is enabled/disabled
- basically, events are hard-coded to certain values, and shouldn't change based on support for any particular subsystem

Fix warnings from clang.

src/common/PKeyboardHandler.cxx

@@ -408,4 +408,4 @@ bool PhysicalKeyboardHandler::handleAltEvent(StellaKey key, StellaMod mod, bool
     handled = false;
 
   return handled;
-}
+}

src/emucore/Event.hxx

@@ -94,9 +94,7 @@ class Event
 
       DecreasePhosphor, IncreasePhosphor, TogglePhosphor, ToggleJitter,
       ToggleFrameStats, ToggleTimeMachine,
-  #ifdef PNG_SUPPORT
       ToggleContSnapshots, ToggleContSnapshotsFrame,
-  #endif*/
       ToggleColorLoss, TogglePalette,
 
       HandleMouseControl, ToggleGrabMouse, ToggleSAPortOrder,