Add CMake option to build WITHOUT_X11

We want to be able to build without X11 support even if some of the used
libraries may not work w/o X11 themselves yet or need to be built with
X11 support for other reverse dependencies.

KTOUCH_BUILD_WITH_X11 already exists and is set automagically so far,
but using -DCMAKE_DISABLE_FIND_PACKAGE_X11 will break if any
dependencies list X11 as required in their cmake config.

Introducing this option means there is no behavior change by default,
cmake will just skip finding X11 or adding unwanted features if the
option is enabled.

- Move the checks to root CMakeLists.txt
- Move finding Qt5X11Extras behind "NOT WITHOUT_X11" too
- x11_helper is conditional on KTOUCH_BUILD_WITH_X11

Signed-off-by: Andreas Sturmlechner <asturm@gentoo.org>