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.
Move constructor and move assignment now behaves similarly to Image
(not only the buffer but also size is swapped). Added constructor taking
size + data, reordered setData() parameters to match order in Image. The
old setData() function is now alias to the new one, is marked as
deprecated and will be removed in future release.