AphrontFormCountedToggleButtonsControl

Summary: Like AphrontFormToggleButtonsControl, but with mouseover counters.  These counters let you know how many of each thing there are in each category, which is useful when using this control for filtering a list of things in multiple dimensions.

Test Plan: `/uiexample/view/PhabricatorCountedToggleButtonsExample/`

Reviewers: epriestley

CC: aran, Korvin

Maniphest Tasks: T2094

Differential Revision: https://secure.phabricator.com/D5118