"Statistics" plots for scheduler GUI

In !357 (merged) a way of plotting Matplotlib things in a separate window was introduced. It would be convenient if execution time plots were updated after changing the schedule.

• Mechanism for updating plots after schedule changes !409 (merged)
• Menu entry for different operations in SFG/Schedule !370 (merged)
  • Method to get different operation types in SFG/Schedule !369 (merged)
• Menu entry for showing memory variables !370 (merged)
• Right-click on connections to show memory variables
• Menu entry for histogram for read/writes (stacked bar plot?) !407 (merged)
• Menu entry for histogram for total storage
• Menu entry for histogram for execution time per operation