Prevent text selection on tabs (Lumo)

[jouni]

This change is 

[web-padawan]

LGTM. Can potentially prevent selecting text by accident on mobile. On the Android Chrome I was only able to reach this using long touch on not selected tab, which does not activate it.

[web-padawan]

@jouni FYI in other components we have (probably inconsistent) un-prefixed version + -webkit prefix, but no prefixes for -moz and -ms:

https://github.com/vaadin/vaadin-checkbox/blob/e6af29c04e5eec119f4b40a0c14a650479b6b161/theme/lumo/vaadin-checkbox-styles.html#L22

https://github.com/vaadin/vaadin-radio-button/blob/9bb42bd4e9556d3b523ab9c761cb6eddaefc2ceb/theme/lumo/vaadin-radio-button-styles.html#L9

I think it is good idea to align those. Will do it on master where appropriate.