Save/load code for the Dead Time filename.

Refs #6691
Don't reset file to empty when selecting "Dead Time from disk".

Improves the UX in the way that if we have not changed the filename,
it will always contain what's saved i.e. previously applied.

Refs #6691
Return the old behavior of clearing input.

Not clearing turned out to introduce more problems. The old behaviour
is fixed as well, as before it was just setting the text without updating
the file cache.

Refs #6691
Save the file even if it doesn't apply to the current data set.

Refs #6691
Close all MDI windows explicitly when exiting.

Do it at the point when MantidPlot is still alive to avoid segfaults.

Refs #6691
Merge remote-tracking branch 'origin/feature/6691_muon_save_deadtime_file'