DefaultRecentFileService: add dispose method

[frauzufall]

I ran this loop when working on memory issues in SCIFIO:

for (int i = 0; i < 1000; i++) {
	SCIFIO scifio = new SCIFIO(new Context());
	scifio.dispose();
}

It was very slow in comparison to creating the Context with specific services. I narrowed it down to new Context(RecentFileService.class)) and this patch seems to make the RecentFileService clean up properly on dispose.

[ctrueden]

Good catch!