Add GLTexture and Cubemap

Some of these changes might break compatibility (e.g. this changes the TextureData interface), but I think not many users would notice this.

https://github.com/xoppa/libgdx/commit/0136754464763ee08c5bd4391ff6b0436f11b99e: Add GLTexture: a class that encapsulates an OpenGL texture handle. It does not add much functionality other than binding the texture (to a target specified in the constructor) and keeping track of it's context (filter/wrap). It also contains some static methods to create an OpenGL texture handle, create TextureData and uploadImageData. I also moved setEnforcePotImages etc. to this class (because its used to upload image data).
(I first had this class abstract (named BaseTexture), but later realized that it can actually be useful without extending it).

https://github.com/xoppa/libgdx/commit/3e645be6787430ab13f0358712e41daa12bd1d05: Add Cubemap and make Texture extend GLTexture. You probably want to double check this one.

https://github.com/xoppa/libgdx/commit/b830ff0a205b9c5572add190c457d797c23f35fd: Attempt to add to GWT, but I'm pretty sure this is not correct/complete. Being a noob at GWT and unable to actually test GWT, please help/guide me in any changes that need to be added.

https://github.com/xoppa/libgdx/commit/ef03f64b7f5c246409dcedffde080ed4a94b1e20: Use GLTexture instead of Texture in TextureDescriptor and TextureBinder.

Tested on android and desktop.

As for Cubemap: not managed atm (no file format/loader for it atm, perhaps we can add something like dds for cubemaps/mipmaps). Mipmapping doesn't work on desktop atm (it does on android), need to look further into that.