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.
Encourages vectorization and generic usage even more. Some functions
were rewritten to make use of the new features, resulting in shorter and
more readable code. This also fixes the annoying naming collision with
WINAPI Rectangle() function.
The old Rectangle is now subclass of Range2D, is marked as deprecated
and will be removed in future release.