Code cleanup.
3D textures _are_ available in OpenGL ES 3.0.
Update OpenGL support.

Don't be too optimistic :-)
Doc++
Split Texture into TextureArray, MultisampleTexture and RectangleTexture.

Each texture has slightly different usage requirements and having
everything under one generic class is not worth the additional runtime
checks and whatnot. The current way with Texture::Target enum
(hopefully not too widely used) is now deprecated and will be removed in
some future release. However general Texture1D/2D/3D usage is not
changed in any way.
Better texture attaching in Framebuffer.

The previous way was half-working at best, as it handled array textures
improperly. Now there is overload for each texture type. The old way
with attachTexture*D() is marked as deprecated and will be removed in
future release.
Shader: ability to use version directive from shader source.
Publicly inherit AbstractTexture in BufferTexture.

There isn't anything to hide anymore.
Allow method chaining also on BufferTexture.

There is only one configuring function now, but still, be consistent
with the rest.
Removed now duplicate extension support status, added some TODOs.