make uninstall

[Alex Merry]

Over on kde-frameworks-devel, there have been several requests to bring back 
`make uninstall` for KF5 and KF5-based projects.

KDELibs4 used to define this for any dependent projects (it's essentially 
`xargs rm < install_manifest.txt` under the hood), and we could easily add it 
to KDECMakeSettings (with an option to disable it).

As far as I can see on LXR, no project that uses ECM defines an uninstall 
target, so I'm not expecting any conflict for existing projects.

CMake has made a deliberate decision not to provide an uninstall target by 
default[0], but that doesn't mean we have to follow suit. Would there be any 
great objections to doing this?

Alex


[0]: 
http://www.cmake.org/Wiki/CMake_FAQ#Can_I_do_.22make_uninstall.22_with_CMake.3F