Time prefs: Set pointer NULL, delete in destructor

... avoiding potential memory leak.
Time prefs: Set text colors in constructor
Time prefs: Use button label symbols & relayout

Add becomes +, Remove becomes −, the current button labels "Add" and "Remove"
are used as tool tips. Relayout the controls so that they fit nicely with the
buttons:

The add button is placed left of the server text control showing that it
adds new server names while the remove button is placed next to the list
view showing that it removes server names from the list.

The reset button is placed next to the "Synchronize" button at the bottom of
the tab group and is renamed "Reset to default server list".

This makes the window a bit taller.