Memory leak in StylesheetWatcher when widgets are deleted

[BryceBeagle]

When a QWidget that is tracked by StylesheetWatcher is deleted from the UI, the StylesheetWatcher does not drop its reference.

For example, deleting a stream removes the VideoSmall widget from the UI. However, as shown below, the Python object is never deleted.

objgraph.by_type('VideoSmall')[1].thread()

Traceback (most recent call last):
  File "/opt/pycharm-professional/plugins/python/helpers/pydev/_pydevd_bundle/pydevd_exec2.py", line 3, in Exec
    exec(exp, global_vars, local_vars)
  File "<input>", line 1, in <module>
RuntimeError: wrapped C/C++ object of type VideoSmall has been deleted

This is especially an issue with the StreamWidgets because they hold references to the SyncedStreamReaders which use a lot of memory holding buffered frames.