Input class

[tomezpl]

An Input class should be built in LepusEngine that separates input handling from Camera code. Currently only the FPPCamera uses window input, but if someone wants to build, say, a platformer, they won't use an FPPCamera and will instead have to write GLFW code in their app to handle input - which is something I want to avoid (the user shouldn't be exposed to middleware used by the engine - the engine should provide all functionality in separate modules instead).