KtFieldNumber: weird cursor position behavior on arrow_key up and down

[carsoli]

Describe the bug
The cursor position is supposed to be always at the end while we increment or decrement whether through typing, pressing the buttons or pressing the arrow key up and down.

It is for all the cases except when we press arrow key up, it resets to the beginning and then to the end in a split second which looks kind of glitchy.

To Reproduce

1. Go to any KtFieldNumber
2. Focus on the field
3. Press arrow key up for the value and compare it with when you press arrow key down in terms of the cursor position behavior

Expected behavior
Cursor position doesn't move to the beginning and stays at the end or always goes to the end