better user feedback

[cytobi]

provide better feedback to the user (apart from debug msgs):

• pop-up/message when the user entered an invalid value (e.g. when creating a graph)
• feedback when creating an algorithm that doesn't immediately do something
• indication which tool is currently in use (e.g. by leaving the button indented

[cytobi]

from tkinter.messagebox import showinfo
# ---
showinfo("Window", "Hello World!")