gui: scroll with middle-button

Hold-down middle button and drag to scroll map without using
scroll bars.  Contributed by askywhale (email, 2013-09-01)

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@850 d9718cc8-9f43-0410-858b-315f434eb58c
MakeTiles: support additional creation arguments

tile_count: to control how many tiles are generated
skip_tiles: to control which tile to start with
MakeTiles: tolerate undefined tiles
android port: convert use of java.awt.Rectangle to own CityRect class