SpinBox, no way to listen for text entered

Hi,

In for example `TextField` you can listen for whatever the user types by means of `onTextChanged`. Your cursor is and remains in the text field with no focus change at all.

In the SpinBox element you don't have that functionality. My usecase for it is as follows. I want to show the current spinbox value in a live way on a slider. The current spinbox is too limited for that usecase because it requires a focus change for values to be updated in onValueModified.

The documentation for onValueModified states:

This signal is emitted when the spin box value has been interactively modified by the user by either touch, mouse, wheel, or keys.

To be clear, the onValueModified signal is not emitted when you type in the spinbox and stay there with your cursor (so no focus change happens). It is emitted when you change focus.