Fix the autocomplete timer activation/deactivation in CustomFileNameEntry

The kKeyUpMessage was received when we opened the open/save file dialog
with a shortcut (e.g. Ctrl+Shift+S), so the timer was triggered and the
autocomplete activated. After that, the text was unselected (as it looks
like fully autocompleted). Now we activate the timer to autocomplete
only when the field is modified (onEntryChange).