Drop the column being dragged at the correct position in wxGrid.

Drop the column being dragged at the correct position in wxGrid.

The column was being always dropped after the column at drop position but this
was incorrect and didn't correspond to the visual feedback drawn by
wxHeaderCtrl: if the drop position is over the "near" part of the column, the
column should be dropped before it instead.

Do the check for this in wxGrid code too now.

Closes #12120.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64475 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775