Changes necessary to make In Place Edit work and look a lot nicer for

Changes necessary to make In Place Edit work and look a lot nicer for
MSW and GTK, and hopefully make it easy to make the same tweaks for
other ports.  There are still some very minor anomalies (changing
colors of the grid doesn't change the IPE control, etc.) but it is
definitely useable now.

The IPE control now gets the fore/background color of the cell.

Changed how the current cell highlight is drawn so mulitple clicks in
a single cell don't leave the highlight in an inconsistent state for
GTK.

Changed a dangerous static variable to a member variable.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@3182 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775