UI: added overlay and setOverlay()
UI: fine-tuned fonts a bit
UI demo: fixed overlay canvas position, modified test resource numbers
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.
UI: now mouse move sets UI.dirty = true
UI: fixed line width of selection outline
UI.redraw(): added "c" as an alias of this.ctx
Merge pull request #34 from gheja/branch7

UI: added overlay, progress bar, menu hotkey handling