Always allow to enter tower placement mode

[basxto]

Currently it is not possible to start placing a tower if you can’t afford it. But…

• it would be nice to see the range of a tower and whether it’s worth saving for it
• if there is only 2 food left, you can’t upgrade a 2 food tower to a 4 food tower.

That should be better handled per cell, by tinting the tower red if it can’t be placed.