- Removed useless window class.
- Removed useless logging to validate event_manager creation and destruction.
- Changed SYSid to be a strongly-typed enum. Cast it to an uint when core logs errors.
- Calculate frame time in the main loop, and report frames per second every 2 seconds.
- Started layout planning in GUI system.
- Created gui::base class, the abstract parent class for all (i suppose) GUI elements. Needs a 'draw' function, but i don't know what else.
- sys_gui now holds a vector of unique_ptr to gui::layouts. 
- Added gui::layout class. Holds a vector of unique_ptr of gui::base elements, thus every element of a gui layout. I guess this is how it should work, no?
- Started tools::shader class, that holds two GLuints, for the shader type and the shader ID.
- Added new files to the CMakeLists.txt.