CORE-20021, CORE-20023, CORE-20037

Complements commits 1786a682 (#7757) and d1aa5998 (#7921)

- When starting eventvwr, the details buttons are disabled and the fields
  are empty.

- During events enumeration, i.e. when refreshing the events list, the
  Previous/Next navigation arrow buttons are disabled. They get re-enabled
  once enumeration is done. No default event item is auto-selected at first.

- When the user selects an item, the details fields are filled (and the
  Copy button is enabled; the Byte/Word mode buttons are enabled only if
  the selected event has associated data).

- When changing the current filter via the TreeView, the events list is
  refreshed, but also the event details fields are emptied and buttons
  are disabled. The navigation arrows are re-enabled once enumeration is
  finished, but the details fields remain empty until an event is selected.

- Pressing the Previous/Next buttons just after the events list refresh
  (when no item is selected), automatically selects respectively the
  last/the first event in the list.
  The rationale for this behaviour is the following: in this situation,
  no item is selected, so it is as if the "current" item is at the same
  time, at the top and at the end (i.e. at the borders) of the list.

- Temporary HACK: When disabling the navigation arrows, this is usually
  because the events list is being refreshed, so we loose access to the
  event's data and we cannot use the bytes/words mode buttons anymore.