Re-added selected and active-drag-cell styles

[ghost]

These were previously removed, readding fixes cell selected and drag cell styles

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/adazzle/react-data-grid/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
Styling around selected cell is missing

What is the new behavior?
Adds these styles back for selected cell and active-drag-cell

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[X] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[coveralls]

Changes Unknown when pulling b5a6c18 on chris-brannen-alps:master into ** on adazzle:master**.

[malonecj]

These were removed for performance purposes. When navigating with keyboard, changing the styles of the selected cell was one of the factors that was contributing to lag. Instead we moved this to use the :focus pseudo class, which now has an outline.

https://github.com/adazzle/react-data-grid/blob/master/themes/react-data-grid.less#L88

Is there any reason this is not sufficient?

[ghost]

I've got two grids on the same page, which both need to show a selected cell and using :focus causes one to lose styling when the other is selected is there a way around this?

[malonecj]

This will be re-enabled in v4 of the ReactDataGrid, when we move Cell selection responsibilities into a dedicated SelectionMask component. You can see progress here