UI: added progress display
UI demo: added progress display
UI: added callCallback() method to wrap around callback calling with the check of current overlay
UI: menu and resources are now hidden when overlay is active, added menu_above_overlay to display the menu above the overlay
UI: changed default menu highlight color to gray
UI: added menu hotkey handling (TODO: mouse click handling for menu)
UI demo: added some menus, fake loader, etc.