Bug 5551 - allow shelf browser filters to be changed

Bug 5551 - allow shelf browser filters to be changed

This adds three system preferences: ShelfBrowserUsesLocation,
ShelfBrowserUsesHomeBranch, and ShelfBrowserUsesCcode. These can be
toggled to determine whether the shelf browser will ensure that the
items being show will match against location, homebranch, or collection
code, respectively.

Note that this also involves a bit of a refactor to clean up the shelf
browser code.

Signed-off-by: Jared Camins-Esakov <jcamins@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>