Don't allow using "-" for unsigned entries in propgrid

The changes of 36f6f8ad49 allowed using
"-" (and also "+") characters even for the unsigned properties, which
hadn't been the case before and doesn't seem desirable, so undo this
part of the changes.

See #1093.

src/propgrid/props.cpp

@@ -162,8 +162,7 @@ wxNumericPropertyValidator::
 {
     long style = GetStyle();
 
-    // always allow plus and minus signs
+    wxString allowedChars;
-    wxString allowedChars("+-");
 
     switch ( base )
     {
@@ -185,7 +184,11 @@ wxNumericPropertyValidator::
             style |= wxFILTER_DIGITS;
     }
 
-    if ( numericType == Float )
+    if ( numericType == Signed )
+    {
+        allowedChars += wxS("-+");
+    }
+    else if ( numericType == Float )
     {
         allowedChars += wxS("eE");