Get around '-' button size on a Mac

.. by making the text Add and Delete rather than '+' and '-'.

   I /really/ hate the way Qt enforces such an arbitrary rule
   for button size and styles, when Apple doesn't.
Ignore dates before 1990 in Model Estimation

.. they're probably bogus or negative or zero and will
   create huge amounts of work !
Only refresh model estimates when needed

.. since its really only the CP history chart that
   uses them, it can refresh them when needed.
Fix gcc compile time warnings

.. in preparation for 3.1 release
.. but still need to fixup for Mac clang too
Merge remote-tracking branch 'upstream/master'