Do not use ‘autorecalculatesKeyViewLoop’ for chooser windows

Instead we manually call recalculatesKeyViewLoop on the window. This is because the auto-recalculation messes up. First time the window is brought up, it works fine, but the second time, nextValidKeyView will return ‘nil’ for the last view (causing the key view loop to not be a loop).
Use bindings instead of notifications for chooser filter string
Use ‘assign’ instead of ‘weak’ when building for 10.7

On Mac OS X 10.7 we cannot use weak references to (amongst others) NSImage objects. In this case, there shouldn’t be a problem with using assign.