Added first UI elements

Rules can be created from the UI
Game can be stopped/started from the UI
Current level can be reset (Load from file again)
Added "Game" target and main file

TODO (A lot):
 - Unit tests
 - Fully functionnal UI
 - Win conditions
 - Level background
 - Level switching/progression
 - Interesting and varied rules
 - Multi-scale pathfinding
 - etc

tests/CMakeLists.txt

@@ -11,8 +11,4 @@ target_include_directories(gTests PRIVATE ${PROJECT_SOURCE_DIR}/../src)
 target_link_libraries(gTests
         engine
         gtest
-        pthread
-        sfml-window
-        sfml-graphics
-        sfml-system
-        pugixml)
+        pthread)