[lmi] wx-3.1.0 regressions

[Greg Chicares]

To reproduce:
  start 'lmi_wx_shared' with any command-line arguments you like
  File | New | Illustration

Desired former behavior: All toolbar icons that are inapplicable for
'Illustration' degrade "gracefully" to grayscale. In particular, the
'edit-class.png' and 'edit-case.png' graphics appear as a dark gray
magnifying glass, on a light gray button background. (Okay, that's
less "graceful" than the behavior of the others, which were copied
from gnome; but it's still somewhat graceful.)

Regressive new behavior: The two buttons cited by name above appear
as uniformly dark-gray rectangles (surrounded by a light gray border).

This is somewhat reminiscent of this old discussion:
  http://lists.nongnu.org/archive/html/lmi/2013-07/msg00018.html
in case that's of any help; but the symptoms here are different.

Supplemental information: This doesn't affect menus, which use 16x16
icons. This testcase is a little harder to reproduce:
  start 'lmi_wx_shared' with any command-line arguments you like
  File | New | Illustration | OK
  File | New | Census
  click the "Illustration" mdi child window
  click the "Census" mdi child window
    [*see note below]
  now, on the toolbar, 'edit-cell.png' displays as a uniform gray rectangle
  but pull down the "Census" menu and 'edit-cell-16.png' is properly grayed

* As for clicking windows above to activate one, then the other...here's
another regression--set up the test as above, up to the '*', then
  Window | Next
  Window | Next
This used to work as expected. With wx trunk, however, "Next" and "Previous"
are disabled on the "Window" menu when the "Illustration" window is on top.