[lnf] Honor the new noModifierGrab in TabBox

Summary:
When the TabBox is in the noModifierGrab mode we should activate an
item when being clicked/touched.

Test Plan:
Tested on X11 and Wayland. On Wayland both touch and pointer
work, on X11 only pointer works - requires further investigation.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5415