Reduce scope of filters property
Reduce scope of filters property

`filters` property doesn't seem to be referenced outside of the
ApisPageComponent it seems to make sense to make it `private`.

Apicurio/apicurio-studio,Apicurio/apicurio-studio,Apicurio/apicurio-studio,apiman/apiman-studio,Apicurio/apicurio-studio,apiman/apiman-studio,apiman/apiman-studio,apiman/apiman-studio,apiman/apiman-studio,Apicurio/apicurio-studio