Dropdown on simulator displays wrong file sometimes

[vandyliu]

Steps to reproduce:

1. Run microbit_file1.py on microbit simulator.
2. Go to microbit_file2.py and save it.
3. Notice on the simulator, it now says it is currently running mirocbit_file2.py when it is running microbit_file1.py

[xnkevinnguyen]

It is currently showing the focused file. We plan to remove the dropdown and have "Currently selected file" (This is the file that would run when clicking run simulator)

[vandyliu]

Fixed in #263