Tracker: Style fixes to DeskWindow
Tracker: Use BPathFinder to find add-ons
BOptionPopUp: check for non-NULL Message(). Fixes #10733

Signed-off-by: John Scipione <jscipione@gmail.com>
Tracker: code simplication to hrev47374

We don't need to create a BNode from a BDirectory because BDirectory derives
from BNode! So use the BDirectory we already have as a BNode and get the
node_ref from it so we can pass it into WatchNode().

Sorry for the noise.