kcms/touchpad: Explicit build options for Wayland and X11 backends

KDE prefers explicit CMake options over a conditional build that
depends on which headers are installed on the user's system.
Two new options are introduced to turn the build for the X11
and KWin+Wayland backends on or off separately.

The touchpad KCM will now build both backends by default, as before,
but won't silently exclude kcm_touchpad from the build if any of its
required X11_XCB/X11_Xi/XORGSERVER dependencies are missing.
Instead, it will loudly fail CMake configuration with an error
and a hint about the existence of the CMake option.

Plus, don't leak in TouchpadBackend::implementation().
If the X11 implementation keeps a backend singleton and returns that,
the KWin+Wayland implementation shouldn't keep creating new backends
that are never deleted.