tiles.rc: dynamically determine number of tiles

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@903 d9718cc8-9f43-0410-858b-315f434eb58c
tiles: keep a mapping of tile names to the tile spec

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@904 d9718cc8-9f43-0410-858b-315f434eb58c
refactor: make TileSpec do its own reference resolving

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@905 d9718cc8-9f43-0410-858b-315f434eb58c
tiles: parse 'building' attribute at initialize time

rather than per-use

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@906 d9718cc8-9f43-0410-858b-315f434eb58c