map edit mode with tile selector
disabled the tile ID tooltip because of glitches
map editor widget to edit terrain tiles
create a load map screen

reusing the existing map chooser
display waypoints and spawnpoints
resource layer selector and editor
don't (lasso) select units in map editor mode
added spawn point placement
generalize actor selector for all units and added tabs
don't spawn harvesters when placing refineries
don't play StartGame notification when loading maps
don't run Lua scripts when editing maps
don't play movies when editing or watching replays

closes #4939
load missions and shellmaps into the editor
categorize shellmaps
fix the shellmap bringing up the main menu instead of map editor