settings-dialog: Fix XtmRefreshRate memory leak
Closes #81 (closed)
Testing
- I am not deeply familiar with the lifetime of these objects, so I relied on Valgrind and AddressSanitizer to detect memory issues
- I verified that the refresh rate can still be changed and is honored by the task manager
- I verified that Valgrind and AddressSanitizer do not report any invalid memory usage when opening the dialog, closing the dialog, and interacting with the dialog's widgets
- I verified that Valgrind and LeakSanitizer no longer report a memory leak in the function