PleineLune theming

Hi.

I continue to code on the stylesheets of the Piwik solution.
In this PR, you will find:

1- Fix regressions:

- Website selector pushes content down instead of on hover
- The language selector pushes content on click
- The language selector right arrow is too large
- Add display of site selector in "Email & SMS Reports"
- The menu should go back to the "Currently selected" menu on focus after abt couple seconds
- In the calendar, bug: clicking on the Month Previous/next button closes the calendar

2- Some refactoring of existing code, like dataTable stylesheets

3- Theme of new elements for the PleineLune:

- Sata tables
- Site selector
- Period selector
- Segment selector
- Widget selector
- About piwik box
- Popin UI
- jQplot graph
- Sparkline graphs
- Transitions

Cheers,
Thomas.