fix(web): support saving focus for custom OSK interactions

Fixes #5944.

If a custom OSK, such as sil_euro_latin, calls `keymanweb.KSF()` or
`keymanweb.saveFocus()`, this ensures that the next focus change is
ignored, which prevents the OSK from being dismissed as soon as it is
clicked on.

This is a regression, probably arising in version 10.0.

Note: `_SelectionChange` is no longer used or referenced, so I removed
it as part of this fix.