global icon theme integration

[Brian J. Tarricone]

hi all-

instead of sleeping (stupid me...), i'm working on globalising the icon 
theme stuff.  i have the ui mcs plugin searching for icon themes, and 
setting Net/IconThemeName in XSETTINGS.  there a couple changes that 
we'll need to make this work completely.  i'd like feedback/permission 
on these.

1) the xfce_resource_lookup functions - we need $datadir/xfce4 added to 
DEFAULT_XDG_DATA_DIRS(?).
2) either
     a) $datadir/xfce4/themes needs to turn into $datadir/xfce4/icons 
(so i can get icon theme dirs with only one call to 
xfce_resource_lookup_all()).
--OR--
     b) all the icons in $datadir/xfce4/themes need to be installed to 
$datadir/icons.
(b) _should_ be ok, but there's the off chance that we're using an icon 
file name that someone else is using in a particular theme, and it might 
not be so nice to accidentally overwrite those, so i think i'm more in 
favor of (a).

hmm, i thought there were more.

also, i have an MCS question.  Net/IconThemeName is in the MCS 
"settings" channel.  how do i join this channel from an McsClient to get 
updates?  if i do an mcs_client_add_channel() i get:

** (xfdesktop:527): WARNING **: Adding a standard mcs channel is not allowed
Client channel not created

without the call, i'm not being notified of changes, so apparently 
clients don't get added to the 'standard' channels by default.  if i 
can't listen on "settings", i'm going to have to poll for the setting 
(which doesn't seem to be working either).

attached is a patch for the UI settings plugin.  may i check it in?

benny: i'm using your xfce_resource_lookup functions, and i noticed 
something that i think is a bug.  xfce_resource_push_path() doesn't call 
_res_init() before pushing the path.  this means that if you call:
xfce_resource_push_path(..., "/foobar");
dirs = xfce_resource_lookup_all(..., "barbaz/");
xfce_resource_pop_path(...);
you end up popping the last of the default directories, since they get 
appended to the path list during the lookup_all.  if i'm right, let me 
know, and i'll commit the fix.

    -brian