
Step1:

just sketching the app: the class structure, the events, the general
idea

Note: this code may be reused. pay attention to `draw_background'

Step2:

adding basic rubber band selection stuff; the selection is recorded in
the corresponding object; try to left click and drag around the
window; for now, if there is another click, another selection is
started no matter what

Step3:

the selection class now gets some code; functionality added: after
releasing the left button, one can now use the left button to move
individual points. a click with the left button outside the points
restarts the rubberband selection
