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.
Optimalizations in Corrade::TestSuite and Corrade::Utility::Debug leaded
to significant reduction of compilation time - on my machine it was
~5:38 before with building of unit tests enabled, now only ~5:00.
Some target platforms supply their own OpenGL headers, thus we cannot
use our own from ES 3.0 and compilation fails.
On the other hand, this will be better for users as usage of unsupported
features will be catched right during compilation and not at runtime.