[lmi] wx-2.8.9: slow toolbar-button repaint

[Greg Chicares]

With wx-2.8.9, certain toolbar buttons seem to repaint slowly.
I never noticed anything like this with wx-2.8.7 .

To reproduce:
  File | New | Census
  click "Run cell" toolbar button

Symptom: The toolbar button vanishes for a fraction of a second.
It's as though the button were removed, leaving just a blank
space on the toolbar filled with the toolbar's background color.
Soon the button is redrawn properly, and the program functions
correctly, but it does seems disquieting that it vanishes.

Miscellaneous: Other buttons that exhibit this behavior include:
  Print case to spreadsheet
  Run case
Buttons that do not exhibit this behavior include:
  Preferences
  Edit [case|class|cell]
It's not simply buttons whose handlers take a noticeable amount
of time that exhibit this behavior: "Edit case" is slow, but its
button doesn't vanish for even an instant. However, the work that
"Edit case" does involves many calls to wx, and perhaps that
affords an opportunity to do a repaint. Buttons that exhibit the
"temporarily vanishing" behavior seem to be those whose handlers
do a lot of non-GUI work.

If that makes sense, then should I call UpdateWindowUI() (or some
such function) before beginning lengthy non-GUI operations?