Add isNormalMouseEvent() to Javelin

Summary:
Added isNormalMouseEvent() that returns true if left mouse button triggered event click, mousedown or mouseup.
Modified isNormalMouseClick() to use new function.

Test Plan: Verified that new function works for click, mousedown and mouseup events.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2475

Differential Revision: https://secure.phabricator.com/D4778