Coerce Table selection to subVI

Table selection forced to be within data bounds. Placing in subVI means
Viewer can share code.
Handle mouse clicks

Only selection should be allowed. On MouseUp, key focus is cleared and
selection size is coerced to table data edges with CoerceSelection.vi.

Created right-click menus; still need to implement copy context menu
event.
Fix selection size bug

On Mouse Down, need to record if click within header (which will cause
LV to do row/col-wise selections). Then on Mouse Up event, bug fix for
selection size by read and rewrite SelSize.

Also added diagnostics to CopySelectedCells.vi front panel.
CoerceSelection.vi also fully nulls selection rectangle if it has no
area.