Media: Properly reset the "add to gallery" library to ensure it accurately reflects the current gallery.

fixes #22815
for 3.5
Nav menu item selection lists no longer need to be specifically de-whitespaced to match hierarchical taxonomy metaboxes. props corvannoorloos. fixes #22954 for 3.5. 
Fix the sprite positioning for HiDPI sorting arrows.

props lessbloat.
fixes #23071.
for 3.5.
Compress scripts/styles: 3.5.1-alpha-23289.
Whenever we have compat fields to render, send a dummy menu_order field (which was always sent in 3.4) to ensure an unchecked checkbox can still be processed by attachment_fields_to_save.

Merges [23290] to the 3.5 branch.
fixes #22868.