Invalidate wxGrid best size when the grid is changed.

Invalidate wxGrid best size when the grid is changed.

Don't keep using the cached best size if rows/columns are added/removed
to/from wxGrid or resized, doing this meant that we always used the first
computed best size which was way too small after adding rows/columns to the
grid.

There could be more places where the grid best size may need to be invalidated
but this should be a good start.

Closes #14761.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72704 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775