Style fixes to LibWindow
Set ScanThread to B_LOW_PRIORITY
Look for system libs in B_SYSTEM_LIB_DIRECTORY

... instead of find_directory(B_USER_DEVELOP_DIRECTORY)/lib/x86

that constant isn't even defined on R1/A4 so I can't imagine how the
code ever worked -- perhaps it didn't.

ToDo: we need to rewrite this to use find_paths() and stop searching
the common lib directory once we can drop R1/A4 support (next release
should be out Real Soon Now, right guys?) so that we also search the
non-packaged system and user lib directories.