Audio: reorder includes.
Math: proper error when using deprecated header.
modules: updated FindCorrade.cmake.
Making includes absolute, part 1: moving files around.

Everything what was in src/ is now in src/Corrade, everything from
src/Plugins is now in src/MagnumPlugins, everything from external/ is in
src/MagnumExternal. Added new CMakeLists.txt file and updated the other
ones for the moves, no other change was made. If MAGNUM_BUILD_DEPRECATED
is set, everything compiles and installs like previously except for the
plugins, which are now in MagnumPlugins and not in Magnum/Plugins.
Making includes absolute, part 2: make it compile again.

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

If MAGNUM_BUILD_DEPRECATED is defined, it behaves the same as
previously, although you may need to recreate your build dir.
modules: updated FindCorrade.cmake.
Making includes absolute, part 4: using absolute includes for Corrade.

Also adapted to some Corrade renames.
Making includes absolute, part 5: absolute includes everywhere.

The only places where they aren't absolute are:

 - when header is included from corresponding source file
 - when including headers which are not part of final installation (e.g.
   test-specific configuration, headers from Implementation/)
Making includes absolute, part 6: better filenames for internal files.

As we are now using absolute includes, there is no need to prefix
everything with "magnum<Namespace>" etc. All generated configuration
files are renamed to configure.h and their path is included _before_
everything else to avoid accidental collisions.
Making includes absolute, part 7: fix documentation.
Shaders: renamed magnumShadersResourceImport.hpp to resourceImport.hpp.

No need to duplicate the namespaces in filename. The old file is alias
to new one, is marked as deprecated and will be removed in future
release.