The existing recentfiles example was basically a clone of the SDI
example with a "Recent" menu added. Assuming it is better to have
it all in one place, the functionality is merged into the existing
SDI/MDI examples.

- Implement recently opened files handling using a submenu and a
  QSettings array in the SDI/MDI examples.
- Remove recentfiles example.

Change-Id: Id5a1ab9fa1c2e6b9ec81309cfe74cf86f450392a
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>