Fix a few minor UI bugs

When using forceFillColumns, resulting table would always be
horizontally scrollable by 3px.

When dragging to reorder columns, header cell text would jump around
a bit depending on cursor position.

Total width of table was being calculated slightly incorrectly - it
was including its own borders (and potentially margin) in the
calculations.
Merge pull request #126 from FellowMD/wiggle

Fix a few minor UI bugs