Removed hard coded value from DetailedProgressViewer used to limit the
number of the entries shown at same time in the Progress view.

The workbench can now be customized (via product customization) to show
an arbitrary number of progress entries in the Progress view. The key is
"org.eclipse.ui.workbench/MAX_PROGRESS_ENTRIES", default value is still
20.

Change-Id: I49ae6524e295558296b04330aabbb7a4087c48f0
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>