Implement custom text context menus to fix crashes

[lhecker]

This works around a bug in WinUI where it creates a single context menu/flyout for text elements per thread, not per XamlRoot, similar to many other areas. Since the XamlRoot cannot change after creation, this means that once you've opened the flyout, you're locked into that window (= XAML root) forever. You can't open the flyout in another window and once you've closed that window, you can't open it anywhere at all.

Closes #18599

Validation Steps Performed

• Flies out right click in the
  • About dialog ✅
  • Search dialog ✅
  • Word delimiters setting ✅
  • Launch size setting ✅
• Across two windows ✅