recent files menu for settings files

. create/update menu with recent files
. connects signal to slot (open_settings)
. open_settings now allows passing the filename into it
. stores 10 recent files
control for number of cpu cores (with ui), refactoring

. ui and logic to set up cpu cores
. refactored recent_settings methods
. refactored skip_dss to skip_sim (same as ui)
. added skip_sim e cpu cores to load/save
. propagated cores parameter along launchers