Client-side text selection cannot be initialised in GridPro

[vaadin-miki]

Text selection setup takes place onAttach, however in case of GridPro the component is not fully available and thus cannot be properly configured.

More specifically, in text-selection-mixin.js, inside listenToEvents(), webComponent.inputElement evaluates to undefined. Since the component is not visible anywhere, server-initiated refreshing has no effect.

This problem was worked around in #274 (and to a smaller degree it is present in #243 - but server refresh seems to work in Grid).