Use garcon for xfce4-settings-manager

[Nick Schermer]

Folks,

Xfce4-settings-manager uses some custom crawling code (using XfceRc)
to get the various desktop entries. Downside is that for example
.local overrides don't work. I'm not sure if we want to support this
(security issue?) but we might be better of using Garcon. Only problem
is that is uses from custom Keys as well (for pluggable dialogs).

Not too hard to work around the latter problem, so the whole question
is: is it worth the effort and so we also want to show other settings
in the dialog (flash, nvidia, you name it).

Nick