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
Fix missing users icon
Add user-generic.png to faces folder, and use it in the new cinnamon-settings
modules, instead of relying on mdm to supply it.
Add admin category in cinnamon-settings, put su programs in it
Merge pull request #2302 from leigh123linux/patch-4

Fix missing users icon
Merge pull request #2306 from mtwebster/su-cs

Add admin category in cinnamon-settings, put su programs in it
Merge pull request #2297 from mtwebster/no-crash-panel

Various fixes.
Sort group tabs in cinnamon-settings-users
Sort groups in user tab in cinnamon-settings-users