wacom: Always try to add the stylus page UI on proximity

This used to be done only if the stylus was "brand new", because pages for previously known styli are added when the tablet is detected/plugged. There are however situations where this may break: eg. the stylus was previously known through a tablet that is not plugged ATM. The tool is however "not new" so no UI is added for it.

We should try to add the stylus invariably on proximity, add_stylus_page() ensures there is only one page per tool anyway.

Closes: #315 (closed)