The problem with the previous implementation was that the table's
$this->prefs can contain valid non-empty value in its default state -
the default column to sort by. On resetting, we must not throw away
these default prefs.
This patch simplifies the TABLE_VAR_RESET interpretation. If such an
HTTP parameter is passed via the request, the table simply behaves as if
there were no previously stored preferences (does not matter if coming
from the current session, or from the persistent cross-session storage).
The logic that decides on whether or not the reset widget should be
displayed is put into a new method can_be_reset() with unit tests
attached.
Finally, the previously private method render_reset_button() is now
protected and the reset widget is given a new semantic CSS class.
