Improve category widget styling

Objectives:

* Get rid of the border and ensure pixel-perfect positioning of the input.
* Always render the arrow on the top right of the category item
* Make the popup positioning based on the arrow div's position and size

Bug 49420
Change-Id: I3ad4ff1c41e6108161b5b7c8718c676eb850de68
Merge "Improve category widget styling"