Impove TextBox selection handling

[rabbitism]

Is your feature request related to a problem? Please describe.
in WPF, TextBox text selection remains opon lost focus, while in Avalonia text selection is cleared.
in WPF there is a SelectionChanged event.

Describe the solution you'd like

1. There should be a property ClearSelectionOnLostFocus, by default false, and allows user to keep the selected text.
2. Add SelectionChanged event

Describe alternatives you've considered

Additional context
In WPF, TextBox selection is kept on lostfocus by default. Although visually it's not, it is selected in fact. There is a property IsInactiveSelectionHighlightEnabled, which controls the visual of selected text. If this is set to true, selected text will be grayed.

[kulikov-dev]

At least it would be nice to have SelectionTextProperty, so we can track it on PropertyChanged event. Right now we have only SelectionStartProperty and SelectionEndProperty.