Primitives: set compile flags for proper library.
Updated and fixed *Query documentation.

Removed misleading "requires_gles30" line for AbstractQuery.
Moved Version enum to its own file.

There will be many places (e.g. all
Platform::*Application::Configuration classes) where Version will be
used without Context (and all GL stuff brought with it).
Added version() utility function.

Converts Version enum value to and from major/minor version number.
Platform: minor code deduplication, documentation and cleanup.
Platform: ability to request context version in Sdl2Application.
Platform: ability to request context version in GlutApplication.
Platform: no-op context version setter in NaClApplication.

Included only for compatibility with other platform toolkits.
Platform: remove #define None.

We need to live without that.
Platform: doc++
Platform: ability to request context version in *X*Application.
Platform: implement ES2 and ES3 support for SDL2 and X-based applications.

Currently XEglApplication and GlxApplication were requesting only ES2
context and SDL2 was not requesting anything in particular (but in
theory should work on ES-only systems flawlessly). Now explicitly
requesting ES3 if MAGNUM_TARGET_GLES3 is enabled and also explicitly
requesting ES in SDL2, so it is usable also with
MAGNUM_TARGET_DESKTOP_GLES.
package: building Sdl2Application also in es2desktop PKGBUILD.
Fix OpenGL ES-related documentation.
Verify that version retrieval went right.

Sometimes the context is created improperly and all gl* functions are
causing errors. Added check for that case.
Fixed compilation of tests on ES3.
Platform: fixed compilation of magnum-info on ES3.
package: added ES3 desktop PKGBUILD.
package: added es3desktop variant to Jenkins CI configuration.
Shaders: fix MeshVisualizer on ES3.