Replace PanelManager for WorkspaceManager

To update the extension with new API of Brackets and to avoid the
message log  "Use WorkspaceManager.createBottomPanel() instead of
PanelManager.createBottomPanel()."

https://github.com/adobe/brackets/blob/master/src/view/PanelManager.js
@deprecated PanelManager module provided for backwards compatibility.  Use WorkspaceManager instead.