Edit Tracker dialog only relies on current selection to modify an entry

[eeppeliteloop]

When editing a single tracker (in the Edit Trackers dialog), the tracker to be modified is the current selection. However it is possible to modify this selection in background since the Edit Tracker dialog (with a single text entry) is not modal. Here are possible bugs:

1. select an existing tracker
2. click on Edit
3. behind the Edit Tracker dialog, select another tracker entry
4. click on OK in the Edit Tracker dialog

The current selected tracker is modified, not the original one.

More important bug:

1. select an existing tracker
2. click on Edit
3. behind the Edit Tracker dialog, remove all tracker entries so there's no current selection
4. click on OK in the Edit Tracker dialog

You get:

    self.edit_tracker_entry.hide()
TypeError: iter must be a GtkTreeIter

So either:

• remember the original tracker name and only modify it if it still exists in the list after pressing OK in Edit Tracker dialog; or
• make Edit Tracker modal (easier)

[Cas]

Fixed: [d7e6afb01e]