Implemented a task progress dialog using a TaskProgressView.
Tasks show up, but without info. Neither the progress nor title and
message are shown.

There now i a progress indicator at the rightmost postition of JabRefs
toolbar. It shows the average progress of all running background tasks.
Clicking it will show a TaskProgressDialog.

still looks ugly and the binding to the average progress does not seem
to be working.

This makes the view work with download tasks for example. Most other
tasks are still shown without title, message (because none are set) and
progress.
Also, there are a lot of tasks somehow.

The progress indicator in the main view still does not work as I can't
get the bindings to work.

The progress indicator is now successfully bound to the list of tasks.
However, tasks do not show up in the dialogue any more.

When using ObjectProperties in the list of tasks, we can use EasyBind to
convert the list into a list of tasks which in turn can be bound to the
list of tasks in the view.

With this, the basic functionality works. There is a progress indicator
in the toolbar that shows the average progress of all running background
tasks. It is indeterminate if any task has indeterminate progress and
shows 100% if no tasks are running.

Clicking it opens an overview of all running tasks and their progress.

Currently, there are many tasks running all the time. The only tasks
that were adapted for this to be pretty are the download tasks, and they
are also still missing an icon.

These are shorter and therefore the task progress view does not need a
horizontal scroll bar.

For consistency with other variables

Make showProgressDialogAndWait actually not only show but also wait.

The dialog that is shown when the user quits JabRef during ongoing
background tasks now shows which background tasks are still running.

When all of them complete or the user chooses to quit anyway, JabRef
quits.

The user can also cancel the dialog. In that case, the dialog closes and
JobRef remains running.

There are still some ImportOrder errors though where I can see no issue.

This makes the dialog that pops up if background tasks are running wait
for pending saves.

This reverts commit d7442cc.

…e/backgroundProgress

Set the pref-width when the indicator is determinate to keep the
indeterminate state from making the indicator wider.

Addresses JabRef#6443 (comment)

More in line with the other JabRef dialogs
Addresses JabRef#6443 (comment)

Addresses JabRef#6443 (comment)

Addresses JabRef#6443 (comment)

Tries to address JabRef#6443 (comment)

Addresses JabRef#6443 (comment)

This reverts commit 0557c67.

This reverts commit 478ee05.