doc: fix CMake variable name.
examples: removed unnecessary namespace prefix.
Making includes absolute, part 1: moving files around.

Everything what was in src/ is now in src/Corrade, everything from
examples/ is now in src/examples. Only moving the files, no other change
was made.
Making includes absolute, part 2: make it compile again.

Added new CMakeLists.txt file, the old include paths are used only if
CORRADE_BUILD_DEPRECATED is set, thus it can now compile and install
only if deprecated build is enabled.
Making includes absolute, part 3: updated FindCorrade.cmake.

If CORRADE_BUILD_DEPRECATED is defined, it behaves the same as
previously, although you may need to recreate your build dir.
Making includes absolute, part 4: absolute includes everywhere.

The only places where they aren't absolute are:

 - when header is included from corresponding source file
 - includes of test-specific headers (i.e. headers not part of final
   installation)
Utility: renamed Visibility.h to VisibilityMacros.h.

The file would otherwise clash with internal "visibility.h" header
after renaming. On Windows platform only, but still.
Making includes absolute, part 5: better filenames for internal headers.

As we are now using absolute includes, there is no need to prefix
everything with "corrade<Namespace>". All generated configuration files
are now named configure.h and their path is included _before_ all
others to avoid accidental collisions.
Making includes absolute, part 6: fixed documentation.