Placeholder text in TextField disappears on TextField tap.

Instead of TextInput.inputMethodComposingtext and TextInput.text.length
use TextInput.displayText.length to determine if placeholder should be
visible.

Task-number: QTBUG-40534
Change-Id: I0de2cb65c7c59d54365172b9233a5aba0f067ff5
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>