[tabbox] Expose noModifierGrab to QtQuick

Summary:
There is a special mode in TabBox which is the noModifierGrab mode. This
is Alt+Tab active without a modifier being hold. This mode is entered
when being activated through screen edges (either pointer or touch). So
far this was not exposed to QtQuick and thus one could not end the mode
using pointer or touch. It is possible to select another window, but not
to activate it. That required the press of a keyboard key.

This setup is rather unfortunate. By exposing the mode to QtQuick we can
react from QtQuick side to it and invoke already exposed functionality to
select and item and directly activate it - existing left-over from the
Plasma Active window switcher.

Test Plan:
Tested on X11 and Wayland with an adjusted lnf package. It kind
of works, but there are additional issues on both X11 and Wayland.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5414