Move X11Cursor into the x11standalone platform plugin

At the same time the xinput2 integration is split out of X11Cursor
and made a standalone part of the platform plugin. XInput integration
is nowadays not only used by the cursor position polling, but also
for modifier only shortcuts.

By splitting it out the modifier shortcuts start to work also when
one doesn't have anything requesting a mouse position polling.

This also simplifies the conditional builds: xinput integration is
only included if we have support for it at compile time without having
to have many ifdefs in the cursor implementation. For the inclusion of
cursor in the kcmkwin this also removes all the ifdefs.

The key events are only requested if we have xinput 2.1. Otherwise we
would not get all raw events if the input device gets grabbed.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2473