Platform: removed deprecated Flags from Sdl2Application::Configuration.

WindowFlags should be used instead, these would now conflict with
context flags.
Follow OpenGL naming in Context::Flag enum.

As usual, the old Context::Flag::Robustness is marked deprecated and
will be removed in future release.
Debug output operator for Context::Flag enum.
Platform: print context flags in magnum-info.
Platform: create context later so its output is at better place.
Platform: don't display limits by default.

The output with limits is overly verbose and I caught myself typing
`magnum-info --no-limits` all the time, so let's reverse the default.
Platform: ability to specify context flags in Sdl2Application.

Finally proper support for debug contexts :-)
Platform: support for debug context flag in GlutApplication.

For some weird reason this doesn't work for me.
package: run the tests multiprocess.

They are now taking too long (9 secs), with -j5 just 3 secs.