Add ability to query dashboard panels by paneltype

Summary: Pretty basic, but you can now search panels by type (query, text, tab).

Test Plan: Searched for a few different types of panels, results look correct

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11782