Resolves: fdo#48723 add a (hidden) CancelButton so ESC closes about dialog

cui/source/dialogs/about.cxx

@@ -77,6 +77,7 @@ AboutDialog::AboutDialog( Window* pParent, const ResId& rId) :
     aCreditsButton       ( this,     ResId( ABOUT_BTN_CREDITS, *rId.GetResMgr() ) ),
     aWebsiteButton       ( this,     ResId( ABOUT_BTN_WEBSITE, *rId.GetResMgr() ) ),
     aLicenseButton       ( this,     ResId( ABOUT_BTN_LICENSE, *rId.GetResMgr() ) ),
+    aCancelButton        ( this,     ResId( ABOUT_BTN_CANCEL, *rId.GetResMgr() ) ),
     aVersionTextStr(ResId(ABOUT_STR_VERSION, *rId.GetResMgr())),
     m_aVendorTextStr(ResId(ABOUT_STR_VENDOR, *rId.GetResMgr())),
     m_aCopyrightTextStr(ResId(ABOUT_STR_COPYRIGHT, *rId.GetResMgr())),
@@ -110,6 +111,9 @@ AboutDialog::AboutDialog( Window* pParent, const ResId& rId) :
     aWebsiteButton.SetClickHdl( LINK( this, AboutDialog, HandleClick ) );
     aLicenseButton.SetClickHdl( LINK( this, AboutDialog, HandleClick ) );
 
+    aCancelButton.SetClickHdl( LINK( this, AboutDialog, CancelHdl ) );
+    aCancelButton.Hide();
+
     FreeResource();
 
     // explicit Help-Id

cui/source/dialogs/about.hrc

@@ -45,3 +45,4 @@
 #define ABOUT_BTN_LICENSE           16
 #define ABOUT_VERSION_TEXT          17
 #define ABOUT_STR_DESCRIPTION       18
+#define ABOUT_BTN_CANCEL            19

cui/source/dialogs/about.src

@@ -122,6 +122,13 @@ ModalDialog RID_DEFAULTABOUT
         Text [ en-US ] = "libreoffice.org";
         DefButton = TRUE ;
     };
+    CancelButton ABOUT_BTN_CANCEL
+    {
+        DefButton = TRUE ;
+        Pos = MAP_APPFONT ( 174 , 6 ) ;
+        Size = MAP_APPFONT ( 50 , 14 ) ;
+        Text [ en-US ] = "~Close";
+    };
     FixedImage ABOUT_IMAGE_LOGO
     {
     };

cui/source/inc/about.hxx

@@ -56,6 +56,7 @@ private:
     PushButton          aCreditsButton;
     PushButton          aWebsiteButton;
     PushButton          aLicenseButton;
+    CancelButton        aCancelButton;
 
     String aVersionData;
     String aVersionTextStr;

unusedcode.easy

-AboutDialog::LinkStubCancelHdl(void*, void*)
 CertificateExtension_XmlSecImpl::setCertExtn(com::sun::star::uno::Sequence<signed char>, com::sun::star::uno::Sequence<signed char>, unsigned char)
 CharAttribList::Clear()
 Dialog::Dialog(Window*, ResId const&)