Restrict movable panels to non-tab panels

Summary: Fixes T12248. Adds a flag for movable panels, and only allows those to be moved. Also cleaned up some CSS rules missing once a panel was drug into a new position.

Test Plan: Try to drag a tab panel content pane, cannot. Drag normal pane, see CSS, grab and drag same panel back, CSS looks the same.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12248

Differential Revision: https://secure.phabricator.com/D17356