Avoid duplicate wxEVT_SEARCHCTRL_CANCEL_BTN event under macOS

see https://github.com/wxWidgets/wxWidgets/pull/698 , along the idea by commit 1776f136f7d673c8e772c9ca6e1607089a071651, implemented a little bit more defensively

src/osx/cocoa/srchctrl.mm

@@ -30,6 +30,7 @@
 
 @interface wxNSSearchField : NSSearchField
 {
+    BOOL m_withinTextDidChange;
 }
 
 @end
@@ -48,10 +49,20 @@
 
 - (id)initWithFrame:(NSRect)frame
 {
-    self = [super initWithFrame:frame];
+    if ( self = [super initWithFrame:frame] )
+    {
+        m_withinTextDidChange = NO;
+    }
     return self;
 }
- 
+
+- (void)textDidChange:(NSNotification *)aNotification
+{
+    m_withinTextDidChange = YES;
+    [super textDidChange:aNotification];
+    m_withinTextDidChange = NO;
+}
+
 - (void)controlTextDidChange:(NSNotification *)aNotification
 {
     wxUnusedVar(aNotification);
@@ -84,6 +95,11 @@
     return matches;
 }
 
+- (BOOL) isWithinTextDidChange
+{
+    return m_withinTextDidChange;
+}
+
 @end
 
 // ============================================================================
@@ -157,7 +173,8 @@ public :
             NSString *searchString = [m_searchField stringValue];
             if ( searchString == nil || !searchString.length )
             {
-                wxpeer->HandleSearchFieldCancelHit();
+                if ( ![m_searchField isWithinTextDidChange])
+                   wxpeer->HandleSearchFieldCancelHit();
             }
             else
             {