Fix missing shortcuts from key bindings.

Correct set WXK_NUMPAD_{ADD,SUBTRACT} for default volume controls and
enable users to change it via menu "Options > Key Shortcuts".

Also allow users to use both these buttons for other input.

src/wx/widgets/keyedit.cpp

@@ -124,10 +124,12 @@ wxString wxKeyTextCtrl::ToString(int mod, int key)
 
 #endif
 
-    if (s.empty() || (key != wxT('-') && s[s.size() - 1] == wxT('-'))
-                  || (key != wxT('+') && s[s.size() - 1] == wxT('+')))
+    if (s.empty() || (key != wxT('-') && s[s.size() - 1] == wxT('-') && s != wxT("Num -"))
+                  || (key != wxT('+') && s[s.size() - 1] == wxT('+') && s != wxT("Num +")))
+    {
         // bad key combo; probably also generates an assertion in wx
         return wxEmptyString;
+    }
 
 // hacky workaround for bug in wx 3.1+ not parsing key display names, or
 // parsing modifiers that aren't a combo correctly

src/wx/xrc/MainMenu.xrc

@@ -334,6 +334,18 @@
         <object class="wxMenuItem" name="SoundConfigure">
           <label>_Configure ...</label>
         </object>
+        <object class="wxMenuItem" name="IncreaseVolume">
+          <label>_Increase volume</label>
+          <checkable>0</checkable>
+        </object>
+        <object class="wxMenuItem" name="DecreaseVolume">
+          <label>_Decrease volume</label>
+          <checkable>0</checkable>
+        </object>
+	<object class="wxMenuItem" name="ToggleSound">
+          <label>_Toggle sound</label>
+          <checkable>0</checkable>
+        </object>
         <object class="separator"/>
         <object class="wxMenuItem" name="GBASoundInterpolation">
           <label>_GBA sound interpolation</label>