Split up TablePager css class, introduce .mw-datatable

Split up TablePager css class, introduce .mw-datatable
* All tables generated with the TablePager PHP class use this css class for their styling. This visual look has since inspired several templates and other tables on the wikis and in extensions to present data with row-highlighting functionality. To make it easier to do that, separating the generic styling into a new css class "mw-datatable" and use it in the TablePager PHP class.
* Kept pager-specific "min-width" rule under .TablePager
* Other CSS classes for TablePager are also untouched
* Changed table-heading color slightly to not be exactly the same as the hover-style (design/usability)