build: Add "uninstall" target

* CMakeLists.txt: Include ECMUninstallTarget.
* cmake/modules/ECMUninstallTarget.cmake,
cmake/modules/ecm_uninstall.cmake.in: New.

This adds an "uninstall" build target.

The ECM cmake modules were copied from KDE's extra-cmake-modules. I have
just changed the copyright notices to GNU style and I have added the
license text verbatim.

• GnuPG-bug-id: T7262