I’ve been working on a high-performance 2D desktop game engine/framework for quite a long time now called oGame (v2 now), also because I’ve found Slick2D to be pretty much dead and LibGDX to just focus too much on being very multiplatform, sacrificing performance and ease of development for having multiple backends and a bit overgeneralizing things across platforms.
Your engine seems to be really nicely written and also quite well documented (which is a rare quality among engines these days sadly 
) so who knows, maybe one time when I get sick of optimizing mine I will start using and forking yours. 
A few minor issues I found while just browsing through the source code: You seem to provide a nice layer of abstraction and a relatively high-level OpenGL access (that GL class is over 2000 lines, lulz) but you still don’t make the use of OpenGL any easier. You should implement high-level objects like FrameBuffer and such. Also, your GUI components are lacking, no font support and the sound features are limited. Instead of just providing a SpriteBatch that renders textures you should probably try to provide a simple entity framework with a batch that truly is capable of batching objects (organizing them by their shader and texture so the engine minimizes the use of graphics calls).
There are some minor OGL mistakes too, like calling [icode]GL.glEnable(getTarget());[/icode] in your texture class constructor (you should only enable texture targets when you use the fixed pipeline, since programmable pipeline you don’t have to/shouldn’t do that).
Might sound nitpicky with some of my points but really all I want is to provide constructive criticism so you can improve the engine. 
