Move cmake related build system to top level

With this merge request the main cmake build system files are moved into top level source dir.

The following files are still located below cmake/:

notes	file
html index file for documentation	cmake/doc/index.html.cmake
cmake find_package support	cmake/DBus1Config.pkgconfig.in
	cmake/DBus1ConfigVersion.cmake.in
	cmake/DBus1Config.cmake.in
config header template	cmake/config.h.cmake
cmake tool chain files	cmake/x86_64-w64-mingw32.cmake
	cmake/i686-w64-mingw32.cmake
cmake checkes	cmake/ConfigureChecks.cmake
default service file	cmake/bus/service.cmake
batch file templates	cmake/dbus-env.bat.cmake
	cmake/bus-test.bat.cmake
cmake modules	cmake/modules

Some of them at the correct place, some of them are also used by autotools (DBus1Config*) and may be moved up and cmake/doc /index.html.cmake may be useful for autotools too.