Made the flipped y-axis on terrain import optional.

Pass a final boolean argument to TerrainBlock::import to control
y-axis flipping. It is enabled by default, since this was the previous
default behavior.

This should be added as an option in the terrain import dialog - see
game/tools/worldEditor/gui/guiTerrainImportGui.gui
Added a checkbox to the terrain import GUI and fixed a space.
Merge pull request #268 from eightyeight/terrain-import-flipped

Importing terrain heightmaps flips Y-axis