mod_admin: Speed up admin edit page JS performance

The save button script was veryinefficient script running on the
window scroll-event; this is now put inside a timeout function.
mod_development: Rescan templates when new one has been found
mod_admin: make pane visible for "Add category"

fixes #591
(cherry picked from commit 54dffcdb9c71084e84c8a239634bf4381ae3af47)