Really support milliseconds in wxLog

Although this was supposed to work, specifying "%l" in wxLog time stamp
format actually didn't because wxLog timestamps were stored as seconds.

Fix this by storing them as milliseconds and add a simple test (not
executed by default) showing that "%l" works correctly now.

Still keep the old wxLogRecordInfo::timestamp field for compatibility,
as it was documented.

See #13059.

interface/wx/log.h

@@ -54,7 +54,18 @@ public:
      */
     const char *func;
 
-    /// Time when the log message was generated.
+    /**
+        Time in milliseconds since Epoch when the log message was generated.
+
+        @since 3.1.5
+     */
+    wxLongLong_t timestampMS;
+
+    /**
+        Time when the log message was generated.
+
+        @deprecated Please use timestampMS instead.
+     */
     time_t timestamp;
 
     /**
@@ -148,9 +159,30 @@ public:
                             const wxLogRecordInfo& info) const;
 
 protected:
+    /**
+        This function formats the time stamp part of the log message including
+        milliseconds.
+
+        Override this function if you need to customize just the time stamp
+        formatting in the log messages.
+
+        @param msec
+            Time to format as the number of milliseconds since
+            1970-01-01T00:00:00.
+
+        @return
+            The formatted time string, may be empty.
+
+        @since 3.1.5
+    */
+    virtual wxString FormatTimeMS(wxLongLong_t msec) const;
+
     /**
         This function formats the time stamp part of the log message.
 
+        @deprecated This function only exists for compatibility, please
+        override FormatTimeMS() in the new code.
+
         Override this function if you need to customize just the time stamp.
 
         @param time