Fix searchbox selection highlighting

[KubeRoot]

Based on TextBox.xaml from Avalonia. Fixes search box not showing any indication on what text is selected.

I'm unsure of why this is an issue, when it broke (or if it was never fixed since the Avalonia port), but it seems like the option to set the brushes for the selection was added in 2019 - I'm assuming this was missed at some point when updating Avalonia. Side note: I noticed the indentation in the file is very inconsistent, but I decided to leave it as is for this and indent the same as adjacent code.

Before:

After: