Adds preset images to preview the generated terrain
Refactors some code for presets in terrain generator
Presets in terrain_generate_modal shown from a list instead of hardcoded

Conflicts:
	app/templates/editor/level/modal/terrain_randomize.jade
Background image paths automated, adds preset image files
Adds an initial dungeon preset, refactors border code to properly create dungeon walls
Fix for flickering thangs due to exact same coordinates
Number of decorations scales appropriately with preset size
Prevents overlapping of randomly generated thangs
Adds undo and redo buttons in editor topbar
Adds rooms and doors to dungeon preset
Adds temporary hack for infinite loops in randomise
Ensures randomised thang clusters do not overlap each other
Fixes bug in undo/redo on SystemsTabView
Updates glyphicons for undo-redo, adds placeholder text for add thangs search
Adds randomize button in case of level with no thangs