Fix style and alignment issues in the DetailsList header

[ThomasMichon]

Overview

This change fixes a couple long-standing issue with DetailsHeader:

• The header cells are missing hover styles. Added theme slots for the hover styles and made them match standard colors.
• If a header cell is narrow, then the status icon (sorted, grouped, etc) overflows the width, causing visual artifacts. Changed the cell title box to use display: inline-flex and made the name truncate in favor of the status indicator.
• Removed the phantom aria-describedby attribute if there is no attempt to provide a tooltip implementation for the header cells.

[dzearing]

@cschleiden just making sure you're aware here.

[dzearing]

@aditima If you're good with this can you merge?

[manishgarg1]

[ThomasMichon]

I mostly was holding this open in case there was an objection to adding a couple more theme color slots.

[hailiu2586]

@ThomasMichon this change causes the contextMenu handler hit area to shrink down to just the text span (.cellTitle). For example, if the list contains a column whose name is 'Id' and the column is wide, the context menu on the column header can only be triggered if the user right click on the text 'id', not the whole width of the column header. This is different from v5.x