The class member IntervalItem::name is redundant since it is a duplicate
of the text() member of the base class QTreeWidgetItem. By removing it
we both simplify the code and remove the need to keep name and text in
sync when renaming and creating intervals.
As a result the itemChanged signal that was connected for renames and
then disconnected when the items are cleared can be connected once
and no disconnect is neccessary.
This connect/disconnect oddity was originally to avoid a SEGV that
resulted from accessing text() whilst the QTreeWidgetItem was being
destroyed. The code for removing intervals when a new ride is selected
no longer destroys and recreats MainWindow::allIntervals (which was also an
artefact of the original code to avoid a SEGV).
I don't like disabling the ride plot tab when I select a manual ride,
because I frequently scroll through the ride list to search for a ride
by the shape of its ride plot, and every time I hit a manual ride I get
bounced to the CP plot tab. So rather than disable the ride plot tab,
just make it show an empty plot.
