nmt_newt_grid_size_allocate() depends on nmt_newt_grid_size_request()
having been called immediately prior, which would normally be true,
except that NmtNewtSection adjusts the label widgets in its border to
match its allocation, so when its size changes, it will end up calling
size_allocate() on the border with out-of-date requisition data. Fix
that by re-size_requesting the border after modifying it.