Improve wxFileCtrl::SetFilename() and SetPath() consistency.

Don't allow specifying the directory in the former and do check for the
directory existence in the latter.

Also update the file shown in the dialog in SetFilename().

Closes #16685.

interface/wx/filectrl.h

@@ -180,9 +180,9 @@ public:
 
     /**
         Changes to a certain directory and selects a certain file.
-        
-        In case the filename specified isn't found/couldn't be shown with
-        currently selected filter, false is returned.
+
+        If @a path includes the directory part, it must exist, otherwise @false
+        is returned and nothing else is done.
 
         @return Returns @true on success, @false otherwise
     */