Store full library path with menus
Add support for recursive directories to GTK library dialog
Load footprints recursively from subdirectories
If there is only one library root node, keep it expanded
Don't recurse into relatively-specified library directories

When PCB is run from the user's home directory and "." is specified
as a library directory (which may be a valid use case), PCB tries to
read the user's whole home directory.  There is no easy way to tell
if a given relative path should be searched recursively or not, so
don't do it at all.
Add support for arbitrary-depth footprint libraries

Tested-by: Kai-Martin Knaak <knaak@iqo.uni-hannover.de>