Thorough CMakeLists cleanup.

 * enable_testing() command needs to be called only in top-level
   directory.
 * Removed (hopefully last) leftovers after QtTest.
 * Removed old forgotten cruft, reorganized the files a bit.
FindCorrade: mark CORRADE_INTERCONNECT_LIBRARY as advanced.

Only CORRADE_INTERCONNECT_LIBRARIES variable should be used.
FindCorrade: don't explicitly include CorradeLibSuffix.

Not everyone needs it and if they do, they can include it themselves as
well.
FindCorrade: fixed formatting with `cmake --help-module`.
UseCorrade: reusable compiler flags.

CorradeMacros.cmake is renamed to UseCorrade.cmake and explicitly
enables C++11 support in the compiler. Additionaly new CORRADE_CXX_FLAGS
variable is defined with all the flags that Corrade uses internally.
UseCorrade: extended CORRADE_CXX_FLAGS (added the ones used in Magnum).

Updated the codebase to not spit out any warnings.
Utility: fixed typo in RESOURCE_FINALIZER() macro.

Not used anywhere, not sure if it is needed at all (resources in
plugins?).
Utility: don't use double underscore as name prefix.
Utility: various cleanup.