TextInput: add API to get and set the selection

[ogoffart]

We should add an API in TextInput (and forward it in LineEdit and TextEdit) to get or set the selection positions

Discussed in https://github.com/slint-ui/slint/discussions/3899

[tronical]

What if we added a get-selected-text() function? That would be straightforward to implement and there’d be no expectation about re-evaluation of the selection changes z

[ogoffart]

if it have this:

VerticalLayout {
  foo := LineEdit {}
  Text { text: foo.get-selected-text(); }
} 

I'd expect the text to change when the selection is changed. (this is trivial to implement) API-wise, i'd probably name it just selected-text and i wonder if it shouldn't be a read-only property.

[tronical]

The setter part was implemented in #4164, but there's no getter yet.