ESP-mode does not always close FileManager threads properly

[dybber]

When opening the file-system pane in ESP-mode, a thread is launched, but we don't close it explicitly and leave it up to the garbage collector, so errors like these can happen:

QThread: Destroyed while thread is still running if the user closes Mu before the thread is finished executing its task (e.g. listing a files can take a long time)

[dybber]

From reading the code it seems this is also the case for the micro:bit mode