Fix keyboard navigation in wxGrid with hidden columns.

Fix keyboard navigation in wxGrid with hidden columns.

The hidden columns (i.e. those whose size was set to 0) should be skipped when
find the previous/next column to select when the user presses Left/Right
cursor arrow keys in wxGrid, otherwise the focus could completely disappear as
it was invisible when it was set to a hidden column.

Closes #13281.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68303 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775