Edit multiple points of the terrain at once

Allow the Terrain Editor to change multiple height points at once. The idea is that when editing the terrain you sometimes want to edit more than a single height point at once, instead you also would want to affect the surrounding height points within a certain radius. For example when creating a large mountain. Currently the TerrainEditAction class has support for multiple movements per action, but only the base point clicked will be moved.
# Add radius and falloff options. The radius (in meters) determines how many base points to affect. The falloff determines how much surrounding base points will be affected.
# Edit the gui to allow for changing of these options.
# Split up the files src/components/ogre/terrain/TerrainEditor.h|cpp into separate files for the different classes, to be places in the subdirectory src/components/ogre/terrain/editing.