modules: updated FindCorrade.cmake and FindMagnum.cmake.
Making includes absolute, part 1: moving files around.

Everything in src/ is now in src/MagnumPlugins. Only moving files, no
other change was made.
Making includes absolute, part 2: make it compile again.

Added new root CMakeLists.txt. 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 FindMagnumPlugins.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 and FindMagnum.cmake.
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 internal headers (e.g. test configuration headers)
Making includes absolute, part 5: better filenames for internal files.

As we are now using absolute includes, there is no need to prefix/suffix
everything. 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 6: fix documentation.