Improve tree placement

Prior to this change, trees tended to either cover the entire map like an ancient forest, or alternatively you turn off their growth which breaks industry. Furthermore there are these ugly random tree clumps at the beginning of the game which look like squares on the map someone placed there.

This change adds a new tree placing setting which removes the ugly random clumps and only slightly modifies the initial placement. The actual growth causes trees to bunch up in higher levels as usual but on the lower 4 levels their growth works differently. The number of trees per tile is limited and the trees spread out over a wider area instead of only to the neighboring tile. That spreads them out more and makes for a nicer look.

This also allows cacti to spread, since they can now use that same algorithm and avoid bunching up, but spread as they should.

src/lang/english.txt

@@ -1543,6 +1543,7 @@ STR_CONFIG_SETTING_TREE_PLACER_HELPTEXT                         :Choose the dist
 STR_CONFIG_SETTING_TREE_PLACER_NONE                             :None
 STR_CONFIG_SETTING_TREE_PLACER_ORIGINAL                         :Original
 STR_CONFIG_SETTING_TREE_PLACER_IMPROVED                         :Improved
+STR_CONFIG_SETTING_TREE_PLACER_PERFECT                          :Perfect
 STR_CONFIG_SETTING_ROAD_SIDE                                    :Road vehicles: {STRING2}
 STR_CONFIG_SETTING_ROAD_SIDE_HELPTEXT                           :Choose the driving side
 STR_CONFIG_SETTING_HEIGHTMAP_ROTATION                           :Heightmap rotation: {STRING2}