Revert supportTabs() removal to fix bookmenu issue

Authored by hindenburg on Dec 27 2019, 4:51 AM.

Description

Revert supportTabs() removal to fix bookmenu issue

This reverts 0995c1dc95bb0df2a04aa84e1034044448ab542e; it is needed
to allow KBookMarks to know we supports tabs and thus show 'Open
folder in tabs' menu item.

FIXED-IN: 19.12.1
BUG: 415549