This patch implements 'move row' and 'move column' features for tabular.
The purpose is to provide a useful behavior in tabular that is
consistent with PARAGRAPH_MOVE_UP and PARAGRAPH_MOVE_DOWN so that the
user can, for example, do alt-<up> to move a row up. Alternatively,
icons for these features are also added to the table toolbar and
context menu.
If there is any selection, the feature is disabled. This is consistent
with how PARAGRAPH_MOVE_UP works in other contexts. Additionally, 'move
row' is disabled if there is a multi-row in the current or target row;
and 'move column' is disabled if there is a multi-column in the current
or target column.
'move row' moves only the left and right borders of a cell along with
the row. Similarly, 'move column' moves only the the top and bottom
borders.
Implementing similar functionality for other insets, such as arrays and
array environments, is on my TODO list.
