Change default argument of grabXKeyboard to XCB_WINDOW_NONE

Summary:
The implementation of grabXKeyboard checks whether the passed in arg
is XCB_WINDOW_NONE and sets the arg to rootWindow. Thus we don't need
to have rootWindow as the default argument, but can have none as the
default arg.

Test Plan: Compiles, test which uses the default arg still passes

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7857