Fix KeysDown not being reset to 'false' when losing focus
Fix KeysDown not being reset to 'false' when losing focus

SFML doesn't send "KeyReleased" event as GLFW and SDL do, so we need to handle FocusLost event to allow for alt-tabbing.
SFML doesn't send the event of "Alt" begin released, so ImGui-SFML thinks that it's still pressed.

Fixes #147

eliasdaler/imgui-sfml,eliasdaler/imgui-sfml