Rename scancode to keycode.

Add `physical_keycode` (keyboard layout independent keycodes) to InputEventKey and InputMap.
Fix non-latin keyboard layout keycodes on Linux/X11 (fallback to physical keycodes).

doc/classes/MainLoop.xml

@@ -26,9 +26,9 @@
 		func _input_event(event):
 		    # Record keys.
 		    if event is InputEventKey and event.pressed and !event.echo:
-		        keys_typed.append(OS.get_scancode_string(event.scancode))
+		        keys_typed.append(OS.get_keycode_string(event.keycode))
 		        # Quit on Escape press.
-		        if event.scancode == KEY_ESCAPE:
+		        if event.keycode == KEY_ESCAPE:
 		            quit = true
 		    # Quit on any mouse click.
 		    if event is InputEventMouseButton: