Workaround Qt regression of no longer delivering events for the root window

Summary:
With qtbase 2b34aefcf02f09253473b096eb4faffd3e62b5f4 we do no longer get
events reported for the X11 root window. Our keyboard handling in effects
like PresentWindows and DesktopGrid relied on that.

This change works around the regression by calling winId() on
qApp->desktop() as suggested in the change. This is a short term solution
for the 5.10 branch.

This needs to be addressed properly by no longer relying on Qt in this
area. KWin already does not rely on Qt for Wayland in that area and is
able to compose the QKeyEvents. This should also be done on X11. It just
needs some more hook up code for xkb, but that's needed anyway to improve
modifier only shortcuts and friends.

BUG: 360841
FIXED-IN: 5.10.3

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6258