Added Canvas (RenderScreen)
- Implemented game loop with variable time step
- Added DisplayModes
- Polished the Engine class
- Polished the Display class
- .gitignore changes (ignores '/.cache' from now on)

Signed-off-by: logineimer <logineimer@googlemail.com>
Implemented rendering
+ Entitysystem
+ GUI (DebugOverlay, will be polished later)
+ Canvas will render every registrated Entity and the GUI

Signed-off-by: logineimer <logineimer@googlemail.com>