GUI: Runcontrol should delete unused writers

When editing low and high limits in the runcontrol dialog, old writers don't get deleted/closed when the dialog is closed, or when the instrument is switched. They only get closed when the GUI is closed and new writers keep getting added as the runcontrol settings are changed.

This is a small memory leak -- these writers should not persist when the dialog is closed.

Start with the runcontrol dialog's writer creation and lifecycle, focusing on what happens when low and high limits are edited, the dialog closes, or the instrument is switched. Verify that unused writers are closed and no longer retained after those actions, and that changing settings does not keep adding writers.

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