Wrote gui module export header (export.hpp)
Implemented empty GuiWindow class

This is a temporary class which encapsulates all issues regarding the window module which needs to be refactored and not-taken-yet decisions.
CMake compiles now the gui module
Created the gui module main header
Added user_interface example
Added GTK2 depedencies
GuiWindow shows up an empty GTK window
Implemented Widget class
Widget has a position and a size
Implemented onSizeChanged event
GuiWindow inherits from Widget and is resizable
Implemented Button widget
Fixed GTK dependencies (GTK2 -> GTK3)
Implemented Container widget
Removed useless namespace prefix (sf::)
Implemented the internal button widget
Implemented getWidgetHandler() method
Container widget adds/removes widgets and GuiWindow is now a Container
Removed useless include from Button.hpp
GuiWindow allows to position widgets at fixed coordinates