CSS adjustments for the "more filters" sub-menu

Some CSS changes for the "..." option in the message list filters:
* Border, colorsand spacing  to meet the header border.
* Position changes to align options vertically with the "..." label
* Extra padding to avoid the menu to be hidden by diagonal movement of the cursor.

Change-Id: Ife50941c6fd4d98d40b1e27a2d80d87593c523c1