Don't try to manipulate the panels before they've been parented.

Was causing cinnamon to crash when panel-edit mode was on during a
cinnamon restart
Fix right-click removal of applet during panel-edit-mode
Add user-generic.png to faces folder, and use it in the new cinnamon-settings
modules, instead of relying on mdm to supply it.
Merge pull request #2297 from mtwebster/no-crash-panel

Various fixes.