macosx: Fix startup delays by loading Open.nib only on demand

There is no need to already load it here, not every user uses the open panel.
Additionally, input devices listing may take one or two seconds the
first time after the system started, so this is also better done only when needed.

close #11098
macosx: avoid ugly delayed initialization calls for the open panel

These selectors may not be called in time as the main loop is blocked
if a modal dialog is open.
macosx: load core dialog nib on demand
macosx: simplifications