"Next Slot" control mapping

-added "Next Slot" to control mapping menu
-added VIRTKEY_NEXT_SLOT -> incremented VIRTKEY_TOGGLE_FULLSCREEN
-added SaveState::NextSlot(); as replacement for code in
WndMainWindow
-added entry in EmuScreen using SaveState::NextSlot();

Common/KeyMap.cpp

@@ -611,6 +611,7 @@ const KeyMap_IntStrPair psp_button_names[] = {
 #endif
 	{VIRTKEY_SAVE_STATE, "Save State"},
 	{VIRTKEY_LOAD_STATE, "Load State"},
+	{VIRTKEY_NEXT_SLOT,  "Next Slot"},
 #if !defined(_WIN32) && !defined(USING_GLES2)
 	{VIRTKEY_TOGGLE_FULLSCREEN, "Toggle Fullscreen"},
 #endif