Add a visual indicator to tabs containing code (issue #1194)

[tstenner]

FlatNotebook-tabs don't allow styling, so this code adds an asterisk after the tab name if the associated CodeBox isn't empty (see issue #1194).

Example:

The titles are only recalculated when opening the dialog, so switching between tabs after editing doesn't add the asterisks. This shouldn't be a problem, as it's mainly needed when opening the code dialog for the first time.

[coveralls]

Coverage decreased (-0.004%) to 50.669% when pulling 3e14244 on tstenner:codetabs_indicator into d2a974c on psychopy:master.

[codecov-io]

Current coverage is 46.43%

Merging #1204 into master will decrease coverage by 0.01%

@@             master      #1204   diff @@
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  Files           231        231          
  Lines         36550      36553     +3   
  Methods           0          0          
  Messages          0          0          
  Branches       5639       5640     +1   
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- Hits          16979      16975     -4   
- Misses        18022      18031     +9   
+ Partials       1549       1547     -2   

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