Always-on scrollbars cause FlexTable headers to become slightly misaligned

[bvaughn]

Steps to reproduce:

1. Create a FlexTable where the 1st column is set to grow and subsequent columns are fixed-width.
2. Set OS scrollbars to be always-on: System Preferences > General:

Table headers become offset by the width of the scrollbar:

This can be seen by rearranging the column orders in the FlexTable example:

Possible solutions:

• Add componentDidMount check to determine scrollbar size (offsetWidth - clientWidth) and subtract from width of table row when scrollbar is visible.
• Implement fixed-headers in Grid component (as discussed on version 5 roadmap) and use them instead.

[bvaughn]

It's not clear to me how common this use-case is and if fixing it is worth the effort.

[bvaughn]

Fixed via PR #81