Do not run cleanup while printing is ongoing.

During investigation of #4994 it became clear that one possible problem with fonts in print is that while printing is ongoing a font is removed and reloaded. If cleanup was set to a low number the problem disappeared.

the cleanup was scheduled when no page or thumbnail was rendered. Because printing uses a different mechanism, t could happen that printing occurs at the same time as cleanup. This is consistent with the observation that the error was reproducible after a few tries (most likely just hitting the 30 second mark).

The solution is to not run cleanup while printing is ongoing. Therefor a flag is set and renderHighestPriority() invoked before and after printing.

For completeness, this PR also nulls the timer when it triggers.
Remove this line, otherwise timer will not be canceled by renderHighestPriority()